Everyone is talking about reasons why using iPython can be impractical, or cumbersome/not-possible to install, but the important thing is that the default experience should strive to be the best experience.
Python has a pretty good standard library, and a pretty good batteries-included toolset, but talk to anyone who's experienced in it and you'll hear about all the Best Tools you have to install in order to be up to par: pip, virtualenv, ipython, third-party libraries to supplement what you want to do (requests, *-parse...).
When the community agrees that there are certain extras that are all good to install, it shows that the features they offer should probably be provided by default. I like Python, and I want Python to provide the best experience by default.