It takes more effort to homeschool than farming it out to public schools, so you're naturally selecting for involved parents. To the detriment of public schools, it should be noted.

It also skews the perspectives of homeschool parents into thinking "this is the best system, why doesn't everybody do it?" The answer is, of course, to take the good aspects of each system and make the public school system more viable again, but there are too many entrenched interests on both sides for this to be easy.