Across the desktop market, you have Linux chipping away at the low end of the market & Apple chipping away the high end of the market.
If Linux developers were capable of producing a single desktop environment, you’d see higher gains for Linux.
1. Linux is an OS. It's not a desktop environment. The DE runs on top of the OS.
2. Linux is known for being able to conform itself to the user's way of doing things--not the other way around. One DE would destroy that, and drive away a lot of users.