There wasn't much point pushing this before KDE-2.0, because for scripting you need applications talking DCOP first. Still a lot of applications want to be ported to the new KDE 2.0 API and make use of the new features provided there. You can't compare this to commercial open source projects like Mozilla or the Eazel file manager. KDE gets written because its authors want to write it, while those commercial open source projects get written because somebody senses business and pays programmers to do the job. Don't get me wrong, that's a perfectly sane thing to do, but it also has an influence on the choice of tools.
Free programmers work for fun.
Better tools promise more fun. If Netscape used Qt, they would have release a modern cross-platform browser two years ago. Now we are still waiting for a final release of Mozilla and what we will get ships with its own middleware, a new component system and yet another widget set. Compare Mozilla with Konqueror, compare the sizes of the development teams, the time they used and the results. Then judge for yourself.