Like you indirectly indicated the i18n coverage of KDE is really huge, nevertheless it's one of the most often ignored features of KDE as a project. http://i18n.kde.org/stats/gui/trunk-kde4/toplist/
If I needed to pick one single reason for a Qt app joining as an official part of KDE I wouldn't pick KDE's framework, I'd pick KDE's i18n and all it's maintainers who make this positive crazyness possible.
This area of KDE need much more publicity...
Like you indirectly indicated the i18n coverage of KDE is really huge, nevertheless it's one of the most often ignored features of KDE as a project.
http://i18n.kde.org/stats/gui/trunk-kde4/toplist/
If I needed to pick one single reason for a Qt app joining as an official part of KDE I wouldn't pick KDE's framework, I'd pick KDE's i18n and all it's maintainers who make this positive crazyness possible.