1. The kernel user base is much smaller in size and far more technically minded than the user base for a desktop environment. It is much easier to keep happy
3. None of the companies pouring money or man-hours into the kernel are providing a commercially supported Linux desktop targeted at consumers. I'd guess that a lot of the kernel developers work on things that are of little importance to the FOSS desktop
2. If Linus wants to write his own awesome DE from scratch, then he should feel free. I don't think desktop users would complain