Python 3 made significant changes in the language which made it incompatible with the ton of software written for 2.7 and this was not accepted by lot of people who use Python so they prefer to stay to 2.7. In this video at PyCon 2013 US Brett Cannon goes over the differences between 2.7 and 3.3 along with benchmark information to show where Python 3.3 shines in comparison to Python 2.7 (and vice-versa). His goal is to convince you that switching to 3.3. is for good 🙂
olimex 
Jul 08, 2013 @ 10:02:03
Right. People never stop underestimating the consequences of deliberately causing a compatibility rift that one cannot pass, only jump over. If color broadcasting would have been incompatible with B/W, mark my words, we’d still be watching B/W TV. Oh, and I’m sure we’ll eventually move over to IPv6, any decade now…