You spend a minuscule amount of time with Dialogs. You spend Most of the time in the project window itself, the mixer, transport, and all the plugins. None of those use standard controls. If they did, they would've pleased all blind users (assuming they do implement the accessibility features you mention), but turned off everyone else because of the bland, ugly and difficult to use UI, not to mention that there are only a very limited set of standard controls. I doubt Reaper would be the success it is now, if it were so.

Having said that, hey, this is Open Source! Like Reaper, I am willing to support accessibility if you, or anyone else is willing to do the extra effort.