This is speculation, but: Google is paying some people actual money to work on Unladen Swallow. People who have made bold claims about the performance improvements, which, apparently, were not entirely justified.<p>I imagine that it would be politically desirable to show success by getting officially blessed, and it would also mean that the project wouldn't just stagnate if the people now working on it were told to do something else.<p>More optimistically, they acknowledge that more coders would be welcome, and hope that it's easier to find those after being included in the main repository. It would also give them an edge over PyPy, their sort-of-competitor.