There is still FUD about the licensing, decades on. And a lot of Linux vendors make Gnome the default - perhaps partly because it's less configurable and so easier to support, but partly it does seem to be this weird prejudice. I wonder whether it's a US/EU thing - most of KDE seems to be developed in the EU and the major European distros (which is only really SuSE these days now that Mandriva doesn't exist any more) seem to have it as default, whereas the American distros seem to prefer Gnome.

KDE is resource hog. And it segfaults pretty often. I used it for year, but I'm returning to something GTK based. (not GNOME, of course)

Not my experience at all (except on Ubuntu, which seem to somehow mess up all their KDE packages)

Don't forget Reconq web broswer. It was pretty faster compared against Firefox and Chrome, and was using WebKit. Sadly, died...

I think it's because Qt has traditionally had a pretty weird dual- or tri-licensing model: https://www.qt.io/faq/#_Toc453700684 which resulted in less adoption by developers because of the viral nature of the GPL.

IIRC, it used to be that even the core Qt libs were GPL (unless you paid for a commercial license), while now most (but still not all) libs are also available under the LGPL.

GTK has always been plain LGPL 2. Nothing scary.

It's slowly coming back. Wireshark used QT in its latest release. Some people are switching back due to GTK3, which is...eh...just eh.

Don't want to diminish KDE, but how is that incredible considering that for each of these there is a much more popular GTK or even Gnome project software?

Hmm, is there?

KHTML/WebKit: I don't think the GTK/Gnome folks ever wrote a browser engine. KDevelop: Did the GTK/Gnome folks ever write a significant IDE with a code model? Krita: It's focused on painting/illustration and doesn't directly complete with Gimp. Among artists doing this Krita is likely more popular than Gimp now, since the latter isn't even in the running. (Support for MyPaint brushes is progress on that front, though painters want many other features.)

For KMail/Kontact there's Evolution, but arguably neither ever became really popular if you compare them to, say, Thunderbird.