Quite some time has passed since TortoiseHg decided to go for GTK back then.
An open source project is also about attracting developers. Since we all
decide for ourselves what we hack unpaid for, we have to make sure we
are using an attractive platform. The pendulum seems to swing to the Qt
side right now.
The GTK based code is still there and I guess if anyone wants to work on
it they can do it. But the current major TortoiseHg contributors seem to
agree on pursuing the PyQt track. I guess I can safely say we are a bit
fed up of GTK.
At least we don't seem to be the only mercurial GUI that switches from
GTK to PyQt: apparently, hgview has done the switch too and other DVCS
GUIs are Qt based too. So we are not alone, which sometimes is good hint
for not being completely on the wrong track :-)