I sympathise, but for me the simple answer would be: because Python 3 support doesn’t solve a problem I have. It’s one more hurdle to leap over in terms of developing, documenting and testing software. And if you’re still using 2.x for production purposes then you don’t need 3.x. Other users might, but there’s little enough time to scratch one’s own itches let alone scratch other people’s for them.
On the plus side, I’m trying to keep my own modules Python3-friendly, running Python 2.x with the -3 flag, making a few decisions which I know will help 2to3 out (or will remove the need to use it altogether). But at the end of the day, the things I write need to work for the environment I have — which at the moment is 2.7 — and not for the environment I’d like other people to be running.
[UPDATE: Thanks to Mike Driscoll for pointing out how to link to a tweet].