Quite a bit late to this, but a major reason would be because he is someone that would have actually had Poe's respect, to begin with. And he's been shown onscreen to know what he's doing, and a familiar character, and... you know, could have done what Holdo did, except do it better and in a way that would have provided the 'Poe is a hotheaded idiot storyline' instead of the 'Poe will make some damn leadership if none is provided' thread we got.
The problem is Holdo had far too much importance and build up to be a throw-away character, yet was treated as such. Bringing her in just for a single "lesson" for Poe then immediately killing her off just weakens the entire film. As the Poe arc is timed to line up with the "Twist" in TLJthere frankly isn't enough time to develop an entirely new character to fit the Antagonist role for Poe, at least not properly. Thus the role kinda needs to go to a decently important character already, preferably as their swan song or their introduction.
That is why Ackbar keeps cropping up as the person who should have taken Holdo's place. It's not bigotry or the dislike of the character, it's just that Ackbar fits the role better. He at least has a reputation prior to TLJ, thus there's enough additional characterization to carry the antagonist.
This is also why the people who were up to date with the Disney EU Books as Holdo was first introduced to them outside the movie, giving her some much-needed depth. That said, I still count this against the movie as works should be able to stand on their own, or at least only on their own medium (i.e. prior films can be a requirement for a movie, but a book or video game can't be).
Now Ackbar replacing Holdo was not the only way to fix the arc, and, in fact, is in my mind the weaker way to do so. As people have brought up in the thread here, Ackbar really got most of his fame from Legends and thus doesn't have the same kind of pull in Disney Canon. In addition, I really don't like writing the new characters and content out of the new Canon if I can help it. So the better way would be to keep Holdo as the antagonist, but make sure she survives the film and promoter to a proper secondary character. I don't care how you do it -- have someone else sacrifice themselves or just cut the hyperspace ram entirely (we aren't Star Trek, keep the lore consistent) -- but Holdo surviving and having her own character development in the background would do wonders to make her a proper antagonist. Plus then you have a foil/rival for Poe for the next film.
So yeah, there's my 2 cents on why Holdo being treated like a throw-away character just makes Poe's arc weaker.
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