it's not even a tag you can search on in the master themes list.
I did raise this as a possible tag addition a while back but the issue with an "accessible" tag is - how do you define it? Where do you set the bar? WCAG1? WCAG2? A? AA? And who do you get to audit each & every theme that is submitted with an "accessible" tag? An accessibility audit is not a simple 10 minute job and the theme review team is pretty stretched as it is.
Then - extend the issue beyond the Theme Directory to the users. With all due respect, a user can completely wreck even the most accessible theme and produce a site with all kinds of access barriers very easily. The very authoring & design tools that allow non-technical authors to run their own sites can be used to introduce all kinds of issues. The new custom background is a case in point. I wonder how many decent themes have been "ruined" by foreground/background contrasts that don't get anywhere near the minimum contrast requirements. And don't even get me started on patterned backgrounds!
Can you produce a WordPress site that meets all the access requirements? Yes - it's been done. Is that site actually as accessible as it sounded? No - it isn't. WordPress and the theme can only ever be one part of a pretty complex picture - which makes it that much harder harder to prioritise accessibility at the development level.
I would dearly love to see accessibility given a much higher profile but that's not happened so far (and not for lack of trying). The whole subject - whilst generally popular superficially - is one that people do seem to have problems getting to grips with when it gets down to implementation. There are also so many other issues vying for attention that accessibility doesn't seem to be able to hold anyone's attention for any length of ti...oooh .. shiny jQuery thingy! ;-)
Might I suggest that, instead of trying to persuade core devs to devote spare time to the subject, it might be better to build up a strong WP-access group with the expertise and know-how to promote and submit practical patches?