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.