I would assume that first and foremost it's that, as the old saying goes, the squeaky wheel gets the grease. And disabled people and their advocates aren't squeaky enough.

Cynically, I think there's another explanation...

I think a lot of activism doesn't actually generate meaningful results. To some significant degree, it just serves as something for people to fight over and politicians to fundraise and campaign on.

To serve those purposes though, it has to be controversial - there has to be a basis on which one party can take a stance in favor and the other a stance opposed. And another handy feature of that sort of activism is that it doesn't have to actually be enacted, and in fact, it's better for the politicians if it's not. That means that the ones who supported it can fundraise and run merely on having supported it and on the need to counter the evil other party who opposed it, while those who opposed it can fundraise and run merely on having opposed it and on the need to counter the evil other party who proposed it. And since no money was spent on any program, that's that much more money the politicians can funnel to their cronies. It's basically free publicity with a bit of "Let's you and them fight" mixed in.

And LGBT might as well have been tailor-made for that exact purpose.

But with something like advocacy for the disabled, there's no basis on which either party could dare oppose it, so there's nothing to fight over, and worse yet, if it's proposed, there's no excuse for not passing it, which means they'd have to pay for it, and that's money that they'd rather be funneling to their cronies.

So politicians mostly just ignore it.