Obviously you can't name and shame, but a blog post saying "I have seen X number of website tenders and none of them include any mention of usability for disabled users" would get some notice.
I think what accessibility and diversity both have in common is that the problem can seem overwhelming. Progress needs to happen step by step, organization by organization. We should encourage and compliment people who are taking even small but meaningful steps -- and "shame" the ones who are making excuses for not even trying.
I am seeing UIs that look nice but are difficult to use for a fully able user, let alone a user with some kind of disability.
I don't use Windows so I can't comment on that, but OS X has a lot of support for disabled users. First, VoiceOver is a built-in screen reader and there are a variety of other options to adjust the contrast, cursor size, amongst other things. There's support for visual alerts, mono audio, and subtitles. One can also use switches to control the system if you have them connected.
Basically, Apple spends a lot of effort to make their products accessible. I know that iPhones are popular in the blind community as the VoiceOver support makes it easy to use.
I've found that OS X is far more usable now than the 10.1-10.4 years. So, while the form has improved, so has the function.
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.