I think maybe also from a popularity point of view, Ubuntu is the most popular Linux distro. If your going to port one to your device, port the one that most users have heard about or used before, which in turn increases users using it. I'm sure there are many other linux distros around that are much more suited to tablets, but if you haven't heard about them before, your less likely to try it.

I'm only thinking of this from a different point of view, I honestly have no clue about the differences between distros.

References:
http://www.testfreaks.com/blog/infor...-bakers-dozen/
http://geektrio.net/?p=1404