Apologies in advance for the poor (attempt at) humour, I don't mean to trivialise a real issue, only to ridicule IE9 users.
In all seriousness it's due to the complexity of catering for certain disabilities each of which require entirely different technical solutions, and some of which depending upon OS and various devices can be seamlessly provided for.
I would say a better question here is why isn't there more standardisation on the technology and approach to better serve the needs of people with a disability.
It's not a purely technological issue either, I've never seen a tender for website or application development with any provision for it. And I've seen a fair few.