This is the flip-side of it. As soon as a God gains any prominence, it's a countdown to some major Cult claiming it to be an aspect of their own. For Bylorak, it'd be Taal or Manann. For the rivers, every time one gets popular it starts getting side-eyed by the Cults of Rhya and Manann. It's a bit survival of the fittest at times.
Interesting. The way Tome of Salvation has it, it's a particular characteristic of the Cult of Taal to militantly eliminate lesser local nature cults, while the other Cults are much more prepared to let nature takes its course and more passively assimilate them as lesser aspects or just let them hang around as part of the broader folk religion.
Notably, for the river spirits, as they're apparently manifest as immensely powerful fey in the material world, I wouldn't like to be the priest that tried to mess around with them...