To be honest, it's an issue that Taal and Rhya's Cults face less so than the Gods themselves. People don't have to be super devoted members of certain Cults to go 'Thank Taal/Thank Rhya', and so on.
The power of the Cults, instead, is described as more of an issue of the worship of Rhya deliberately not formalizing and entering into the arenas of politicking and power that the Cults of Ulric, Taal, and Sigmar partook in. Additionally, the Rhyan portions of the Cult as well as the Taalite ones became by necessity sort of removed from the main areas of power and 'importance' in governing terms aka cities and major settlements.
And, as the 3rd Edition tried to explain it, her domains are soooo widespread that she's sort of faded from major sight for a lot of people. Not to mention, while she definitely has some origins in herb lore and early medicines, a lot of that knowledge was surely shared with the Cult of Shallya for mutual benefit which would go on to end with the White Dove somewhat supplant Rhya in such matters. I do a mix of different Editions, this should be known by this point, with 4th Edition being used particularly sparingly.
Tome of Salvation notes that the Cult of Taal and Rhya has their Cultists usually spend their time wandering alone, off in the wilderness or in tiny groups living off the land. And while their influence is significant in remote areas, it is 'mainly because they are often the only priests in a given area'. But also, again, it emphasizes that they deliberately do their best to steer clear out of politics and power jockeying, with it being IIRC in canon that they outright refused Electoral Seats when offered by Magnus.
But as to your question as to their domains and prominence, for one, Taal is worshipped/mentioned/acknowledged for a bevy of things as well. He is literally worshipped as the power of all nature itself, of rain, vigor, growth, and so on. He's the original King of the Gods, and while yes he has 'less' people worshipping him and calling on him as the agricultural peasantry do for Rhya, woodsmen, trackers, and rangers all still are major worshippers for him.
And it simply cannot be forgotten that, for pretty much the entirety of the Empire's history, more of it has been dealing with the Great Forest than it has the fields. The southernmost provinces, like sundrenched Averland, Solland, and some of Wissenland, and major portions of the Reikland, have major agricultural sectors and provide places for Rhya to flourish.
Others, like Talabecland, Hochland, Middenland, Nordland, Ostland, portions of Ostermark and Stirland, are PRIMARILY big forests, where traveling a tiny way out of any settlement is immediately frontier territory. Not for nothing is the Empire sometimes described as islands of civilization in a sea of forest and darkness. There is just SO MUCH forest and therefore people who have to deal with said forests. So Taal gets all that prominence, plus the historical fact that Talabecland and Talabheim are called that and not Rhayabecland and Rhyasheim.
Quite literally, there is more for Taal than Rhya if we're focusing exclusively on a few of their domains, with Taal's being big major ones as well as Rhya. Rhya is more diminished in the eastern and northern portions of the Empire, whilst she is more prominent in the south. But, as the Empire has advanced, sections of Rhya have gotten twisted up/conflated with Taal, with nature and animals and harvests, meanwhile other sections of Rhya have gotten supplanted significantly by Shallya. And, in a more puritanical way, it is not necessarily accepted as 'civilized' to be too open about love/sex/carnality etc.
On a different cultural note - temples!
They don't uh, really do those. They preserve places of natural beauty, take to waterfalls, open glens, a few circled stones, some wells and springs. There are occasional shrines, but the major temples that just about almost every other Cult likes to go for isn't there. Which, quite frankly, means that there are not as big major congregational places for them to pop up into and make big shows of their religiosity and presence in society outside of, like, Talabheim and the forest within the crater. And regardless of whether someone thanks Taal or Rhya for something out in the fields or on the road, there is a good portion of the populace, especially those who make their whole lives in mostly urban environments, who likely feel a bit disconnected from a Cult that is so different from any of the others they are familiar with. Which makes them strange, and disconnected from them the potential worshipper.
Plus, the Daughters of Rhya, the lesser order which specifically focuses itself as midwifes/healers/counsellors solely for mothers and mothers-to-be, actually hold themselves somewhat aloof and secretive. Secrets like how to avoid getting pregnant, dealing with bad husbands, and yet are often disliked by more patriarchal Cults as a result. So they keep themselves in the background on purpose.
But the Bringers of Bounty retain a place, most respected and accepted, for their aid in hunters/livestock/crops/etc.
But the point, again, is that they are sought out for practical knowledge as much as blessings, and that's what it comes down.
Practicality and less desire for being as big and open and 'powerful' means that they are diminished/dismissed more by those in 'power' as a result of simply not engaging. They are incredibly widespread, acknowledged, noted, prayed to, whispered to, and so on. But it is without the major cathedrals and temples and mass crowds of worshippers going to service every day, it is every farmer or hunter murmuring a prayer on their lonesome or in small groups before they get on with their job for the day. This not like the Cult of Manann, who is secretly the soft 3rd most powerful Cult in the Empire and more widespread than any other in the whole of the Old World due to - you know - all oceans and rivers and seas and such, who are connected in a powerful web and hierarchy. The Cult of Taal and Rhya are specifically separated, distinct, and broken up into two separate Hierarchs per province, with no 'superiors' or higher priests beyond that. Hierarch is as high as it goes, and it goes at the provincial level.
At the end of the day, it's not a matter of prominence or presence, because both really are quite prominent and present in the commoner's life out in the Empire, in the fields and forests, even if in a much reduced manner in the cities and other major urban centers.
It's a matter of active visibility, to which neither makes major efforts at, but quite simply Taal is somewhat more...easily visible. You know, with the province and big city and also the big hunts bringing in the furs and meats for everyone and all that. Rhya gets her due, of course, but Taal just gets a bit more than her in most interpretations of their mythology. He's the powerful raging embodiment of nature, she's the one who calms him down. And, while portions of the Empire are more egalitarian, other portions are not so much. That's just the unfortunate truth.
We're more 2nd Edition than 3rd, so at least Rhya hasn't literally become so diffuse that she has ceased to be to the point that it's just the Cult of Taal.
EDIT:
Plus, there's just...sorta less chances/places for mega heug acts of faith and power and such? You really don't see them haring off to challenge the Dark Gods, have whole armies of people praying to them and fighting for them and dedicating incredible world shaking deeds for them. You might have someone dedicate a really big buck they took down to Taal, or the finest head of one's herd to Rhya, but sheer levels of 'potent' fervent major constant active mega worship like other Cults get just isn't part and parcel of what Taal and Rhya get. They get regular low-level stuff, for the most part, which is more than nothing, certainly, but still. Shallya gets play in urban, sea-based, rural, and warfare environments, but you don't really see Rhya or Taal showing up nearly as prominently in big marching armies that feature so heavily in the world that is Warhammer, you know? They've got Longshanks and Horned Hunters, but Manann has war fleets and fisheries and all that other stuff too, plus big storms and water-based wars and rivers and so on - and depending on your interpretations and location in the Empire that means that Taal who originally had something over storms and rivers as well lost out on that to Manann in some aspects.