
They didn't know, but should have known.
A number of people on kde-devel@ made exactly this argument -- calling it 4.0 would mislead people into thinking it was ready for end users when it's not.
It's not enough to say "Yay, KDE 4.0!", and then follow up with "oh, by the way, this is a technology preview/developer release".
Go read the KDE 4.0 Press Release. There is not one mention of the fact that 4.0 is intended for anything but the general user population. Saying it's "the beginning of the KDE4 era" implies nothing about the quality of the 4.0 release; it only indicates there are more features in the pipeline.
If KDE wanted to send a clear message that 4.0 was not ready for anyone but early-adopters, the press release was the definitive place to do it. But they didn't, and all this angst is the result.
PR mistakes aside, I still think that KDE is, and has been, a great desktop. I've been using trunk as my main desktop for several weeks now, and now that 4.1 has been branched, I'm really looking forward to what 4.2 has to offer.
Many people are unenthusiastic about their work.