Companies do their best to MAKE it "niche" though
For instance, the useless one-off crap that comes on eeePCs... why do all that work when it's probably a lot easier to port straight Debian and put Asus logos on the desktop background?
Then there's the crap on the Nokia N800s, where there's proprietary bits scattered all through, especially in the battery handling, GPS, and weird boot sequence. You can't (until recently) upgrade the OS without completely reloading the flash with a new image, losing all your installed apps and customizations, even though it was fully apt-package-based.
Crap like that is what turns it into a niche market. All I want is a small N810-style tablet with a touchscreen and slide-out keyboard and an ARM processor, that runs plain 'ol Debian, with something lightweight for the window manager instead of KDE/GNOME. An eeePC in an N810 form factor.
Companies need to stop the "because we can" one-off proprietary embedded crap.