[forinti]

It's my first choice if I have to build a standalone app with GUI. With freewrap you can build an executable for Windows and the same source works for Linux.

I wonder why it isn't more popular.

[maxerickson]

There is a strong sentiment that Tk is not attractive/not platform specific enough.

(I guess this has been at least somewhat remedied in later releases, but that doesn't necessarily have people taking a second look)