A lot of the points here are valid but I personally think it's partly because disabled people aren't "pretty". There's a narrow band of what's sort of socially accepted as a disability and if you're not in that band you're kinda screwed. If it's not visible enough you're faking or overexaggerating or a hypochondriac. If it's too visible it's gross and annoying and 'why are you even out if you need everyone to cater to you?'. And when it comes to issues and accepting them, I feel like most people mainly care about the "normal" people who just happen to be apart of that group. Your Ellen DeGenereses and captain Holts and whatnot. Think about it - whenever you usually see disabilities in media, it's usually the same set of easily identifiable ones and a lot of the time the character in question has something that negates it in a way and if it is something more nonstandard, it seems like it's the butt of the joke a lot of the time. And that doesn't really work for disabilities because of how varied they are and how they often need conflicting things. You can't just fight for the nice socially acceptable ones and call it a day.

Same goes for mental illness - it feels like most people are still working from the same set of sterotypes where you're either a deranged maniac or an inaccurate sterotype like a savant with no social skills or maybe a hyper idealised version of said condition. And it's hard to fight for accommodation when people don't even understand what you're fighting for.