[whichdan]

Are packages not updating because it's a lot of work, or because updating will break backwards-compatibility?

[raverbashing]

A little bit of both

There are ways to write software that's compatible with both Python 2 and 3 (using six for example: http://pythonhosted.org/six/)

But I guess it's mostly developers not having the time to update (it's easier if you kept it updated and not using some old constructs)