Commenter Maya comes up with a pretty good analogy:
The truly smart people aren't staying away [from teaching] because of the money. The pay is decent enough for a job that is meant to be emotionally/spiritually rewarding and with ample vacation time. The truly smart are too smart to get into a situation where YOU [the media and the public] blame, demonize and threaten them for the severe shortcomings of other people. Smart people would rather become dentists, for example. A dentist doesn't get blamed for his patients' cavities nor is it ever suggested that a man getting his teeth broken in a bar fight is his dentist's fault.
Of course, dentists really can fill cavities, but teachers can't actually make everybody equally smart so that, as the federal law demands, no child is left behind.