Maybe a hint?

It's probably also the sheer amount of stuff in Qt/KDE. I wouldn't be surprised if there's an order of magnitude more code in the Qt/KDE frameworks than in GTK (counting the libraries, not the respective desktop environments). Shipping old libraries is all fine as long as upstream is active or when it's only a small maintenance burden.

On the other hand, Qt upgrades have been relatively painless in my experience. They don't tend to make major redesigns that force you to significantly change your UI or rewrite parts of your app. QML/Qt Quick is 15 years old and that's what they recommend for new projects. But you can still port and build your old QtWidgets app with new frameworks, that's also fine.

If Debian only ships Qt5 and Qt6, it's not like applications should really feel trapped on Qt4. The migration is tedious, but for most apps it's mostly mechanical work. It doesn't that all that much pressure in the first place.
Of course if you look at complex projects like GIMP or Krita the migration will be much more difficult. But even in more complex applications, the trend seems to be borne out. (I expect Krita on Qt6 long before GIMP on GTK4, despite both toolkits releasing in the same month.)