Adding composite features to KWin was less work than to extend Compiz with the missing features. One result is that the work flow stays smooth when KWin is switching from composite mode to unaccelerated or the other way around. In current GNOME environments hacks are used to switch back and forth between Compiz and Metacity, resulting in entirely different decorations – and button orders – being loaded.
Compiz is also heavily tied to desktop technologies. OTOH KDE is also developing an shell for smartphone form factors. KWin is easier to adapt to it than Compiz. GNOME-based Maemo uses the Matchbox window manager for that reason and not Compiz.
Oh and btw, KDE and Compiz play nicely with each other. They are currently jointly developing a composite manager spec. The atmosphere has always been more friendly between KWin and Compiz than Metacity and Compiz.
I have yet to try Mutter but performance of a single window manager is no reasonable reason for switching the whole desktop environment. Canonical could just as well port the necessary features to Compiz and use Shell with it. Since Shell and Unity both use Clutter, the performance is likely similar anyway.
Btw, according to ArsTechnica Shuttleworth stated that GNOME and Canonical don't have the same design philosophy ... that Canonical believes in a unified global menu while GNOME doesn't.
One could be inclined to actually believe that claim if Canonical ever shipped http://code.google.com/p/gnome2-globalmenu/ by default and now made the technical argument that gnome2-globalmenu no longer works with Shell and Canonical was forced to adopt Unity for Ubuntu Desktop as well.
That leaves me with http://ebb.org/bkuhn/blog/2010/10/17...admits-it.html and Canonical's desire to control Unity's copyright in order to allow proprietary variants by OEMs.