They tend to be older and more focused on their education; to have already demonstrated that they can complete 2 years of basic coursework without dropping out; and have mostly completed their general education classes, meaning they are much less of a drain on the institution. None of these reasons demonstrate 'better-than-average' quality of community college education, just the quality of the kind of students that pursue it successfully, and where they are in life when they apply somewhere else.

The very strategy of doing your first couple of years (ie - the general stuff you can teach yourself from the text book) before going somewhere else to your final degree belies the argument that the education is 'better than average'.