Because KDE and Gnome are bloated hogs.

And because, even with a stylus, enough is different that a good UI that's an easy port will be substantially different on the device. There's things you can do without making people rewrite their apps, and those should be done.
But yes, we could have both (a sane UI for desktop apps on a handheld device, and a dedicated UI for apps specifically targeting this device), and I'd like that.
Just don't ditch the ease of porting entirely, especially not in favor of revision 0 of a brand-new UI.