Another reason for this is sadly, that accessibility isn't a wildly known thing. Developers don't care about it for the most part, and if they do, it is never enough, due to the behavior of other developers.
How many times did the orca folks and various associations for the blind have to slap the gnome people on the wrist for taking out accessibility features or causing regression without caring? Too many, that's how much this happens. That's how regular it has become for us to have to fight over every version of gnome released to keep the accessibility more or less working.
Another good example of this is GTK. When GTK 3 came out, there was a bit of fighting over accessibility, but nothing alarming. The GTK folks were generally happy to make the toolkit in a way that it had built-in accessibility support. Default widgets came with accessibility from the get go, a bit like html 5 does nowadays for websites. And it was great this way.