That's a really good question. There are a bunch of reasons, including it's
a pretty complex project. So I'd rather port it to something else at the
same time I port it to Python 3.x, and get through all the pain at once.
And, although I'm sure TG 2.x is better, TG 1.x is pretty nice. I've had
the opportunity to recently work on Django, Pyramid, and Flask projects,
and TG 1.5 still has lots of charm.
Once TG supports Python 3.x I'll probably make the transition. In the
meantime the framework still works really well for me. I can't think of
anything I'm missing. Although I still have to hack up the redirect
function every once in a while to make it do what I need it to. ;)