Because IE9 is the first browser by Microsoft that supports standards sort of reliable like more than 70% of the other browsers out in the wild. So we support it because doesn't take much effort to support it. The choice IE9 or better is because of the quality of that browser, not because we care a lot about the actual marketshare.
Furthermore I always try to optimize the "dry" HTML and use semantic HTML not because of the blind and disabled but because I like my documents be read by non-humans that apparently are blind and deaf and have bad JavaScript support in their browsers to boot.
Don't do it for the disabled.
Do it for the blind and deaf unstoppable corporate robots.
Accessibility is more than just Perceivability - a characteristic robots would appreciate. Accessibility includes Operability and Usability, these are more human focused, and less robot focused principles.
What's the point of having a perceivable interface without it being usable or operable? See this: https://news.ycombinator.com/item?id=7885113 -- for accessibility tips that have no relevance to spiders/bots, but is fundamental to accessibility.
That's where this "robots are like blind/deaf people" argument falls down.
Accessibility is about removing barriers for humans, with human related disabilities. That making those changes also makes content more consumable by a spider or crawler is a positive side-effect. A great positive side-effect, but not the primary aim.
Furthermore I always try to optimize the "dry" HTML and use semantic HTML not because of the blind and disabled but because I like my documents be read by non-humans that apparently are blind and deaf and have bad JavaScript support in their browsers to boot.
Don't do it for the disabled. Do it for the blind and deaf unstoppable corporate robots.