Because ubuntu goes his own way more and more; this increases incompatibility
to other choices.
Think of the whole MIR story. Who knows whether one day, unity will be the
only desktop environment which is able to run on ubuntu?
Looks very bad for kde at the moment; this is what I've heard.
Canonical does nasty things and it will become more and more difficult to
remove those things. One day, It might be impossible, I don't know.
And it's very questionable how long ubuntu will stay the most popular
distribution with their policy.
