I gathered from an article i read recently [and can't remember where
now!]; Ubuntu is the professional version that Canonical try and push,
Xubuntu and Kubuntu are the volunteer versions that the community
develop [sure they are quite good but they are not as polished as
ubuntu]. I don't know why gnome is chosen but i imagine it is because
it is a little more straightforward to develop than KDE. It seems that
a distro is either gnome or KDE and developed along that route e.g.
suse is a lovely KDE interface and not bad for gnome but they
definitely develop down the KDE route, similarly with PCLOS.

I tried a few distros and returned to Ubuntu (i didn't actually leave
it!), and despite a few glitches now and again, i will stick with
ubuntu for the better package management, smoothness and reliability
of the application.