Performance goals have not been met so far; it increases memory usage; PyPy is an alternative with more community support (if only because the typical Python programmer knows more Python than C++); and it makes the code much more complex, introducing lots of hard-to-debug problems, at a time when the developers already have trouble getting people to migrate to 3.x.
The 3.x branch does not need a reputation for instability.
(None of this is to say Unladen Swallow is a bad idea, or should be killed; but it should prove its worth first.)