Because everyone is afraid of a v4, after the 2-3 debacle. And there are things which need to be culled every once in a while to keep the stdlib fresh.

Python is culling stuff all the time, but that doesn't warrant a major version jump.

You are probably right about Pythons careful approach of when to ship v4, but for the wrong reasons. Python 3 was necessary not for the removal of functions, but because of the syntax changes e.g., moving print from a statement to a method.