It's a long story. Basically it's become more and more GNOME specific, up to a point where it has become increasingly complex to use it outside of GNOME. Also migrating from one version to another is vastly harder than Qt, see how lots of applications and environments are still stuck on GTK+ 2 or 3.

On the other hand, Qt is completely independent from KDE and it's easy to use KDE frameworks in non-KDE apps.