Because in public education, they are forbidden from going faster than some bureaucrat (read: democrat) imagines the slowest child can go. No child is left behind, because no child is permitted to run.
I'll take a stab at that. I've seen some tremendous public education programs: McLean and Falls Church, VA, Bethesda, MD and my area's public school system. Not to put a political spin on it - although that's hard to do - but three of the four I mentioned are Republican enclaves with upper middle class families dominating the demographics (the lone democratic one, Bethesda, is awash in money too). I think you can see the points here: one, there is more money in the communities to support public education; two, families are more traditional (two parent households); and, three, the parents were instilled with idea that education (continuous education) is the key to success.
A bit off track from the discussion but nonetheless germane.
A sad, but none-the-less apt characterization of the problem.
Tsk, tsk, who do you think you are to question the Handicapper General?