Building on: http://www.reddit.com/r/Python/comments/7v5ra/whats_your_favorite_gui_toolkit_and_why/
1 - ease of design / integration - learning curve
I'm just weighing in to say that TKinter sucks. It sadly seems that it is packed with Python because of backwards compatibility.
The documentation is horrible. It looks horrible. I have run into some bizarre bugs that will actually crash Python.