Very slow release cycle and long lifetimes. Qt moves pretty quick and this causes problems for downstream projects. E.g. KDE had to keep Qt 5 alive for _years_ until KDE 6 was ready. Rochus Keller has a version of Qt 5 as well. Trinity is still maintaining Qt 3.
Extremely wide cross-platform support, e.g. on the BSDs and some other niche OSes. I think there's even a Haiku version.
It could certainly be wired up to Tk, but what does Tk offer that Qt already doesn't when it comes to a portable cross-platform toolkit?