It's because they stopped development in the late 90s. Before Windows 95 (Chicago) came out, HP-UX with VUE was really cutting edge. IBM kinda screwed it up when they created CDE out of it though.

And besides the GUI, all unixes were way more cutting edge than anything windows except NT. Only when that went mainstream with XP it became serious.

I know your 20 year timeframe is after XP's release, but I just wanted to point out there was a time when the unixes were way ahead. You could even get common software like WP, Lotus 123 and even internet explorer and the consumer outlook (i forget the name) for them in the late 90s.