I think a missing piece of the story is that there haven't been UX designers involved in the development of Linux DEs. MS and Apple have money and motivation to hire UX people and do user testing on their interfaces and they don't even get it right all the time. The folks doing open source desktop environment development are mostly scratching their own itches and so (I think) by definition, developing niche products.

EDIT: For which I am extremely grateful BTW. I love to see experimentation in the UI space. Just pointing out a reason we got here that I don't see mentioned often.