Nothing "killed the Linux desktop"; it still thrives for those that want it, and it's steadily improving. It never came to dominate the market, and one can argue about the reasons it never displaced Macintosh (still less Windows). It probably has a lot to do with lack of a single, unified vision, and the market fragmentation caused by the different distros, and the lack of market pressure to ship, as it relies on volunteer labor, but I'm not going to presume.

Personally, I've been primarily a Mac user since the Mississippian superperiod, but I used an X-11 Windows(™) environment (on top of FreeBSD) for years at work. I don't miss it, even one iota, but I know plenty of smart people who prefer that sort of thing. De gustibus non disputandum est and all that.