Why does Ubuntu use Python 2.6 and not 3.X?

I have recently started learning Python and thus have learned that Ubuntu (10.10) uses Python 2.6. It seems to me that Python 3.X is still rather unpopular in the Linux/Ubuntu community.

Why is that?

Edit April 30, 2012
Python 3.2 is not shipped by default in Ubuntu. There are plans to make Python 3.2 default in 12.10, however, and make it the only version shipped by then. Python 2.7 will then not be on the default ISO anymore. Note: this is only envisaged, not certain yet.

https://wiki.ubuntu.com/Python/FoundationsQPythonVersions

I don't think Python 3 is less popular, it just haven't been around as long..

For a language to be used adoption time is needed, version 3.* is so much different from 2.* that you can't use the modules/extensions without changing them beforehand. Therefore all modules that the current infrastructure depends on needs to be ported python 3.*

"If you don't know which version to use, start with Python 2.7; more existing third party software is compatible with Python 2 than Python 3 right now." -Python.org download page.

Se also: Python 2 or Python 3 and Wikipedia

see http://wiki.python.org/moin/Python2orPython3

Because Python 3 is still under development and until now a lot of libraries are not ported to Python 3.