There's a clear correlation between education and having liberal ideas/values. Even if one doesn't know that, or doesn't believe it, they can still see it in day-to-day life. Also, people who aren't educated seem to feel threatened by those who are. I could go on for ages relating anecdotes from the past 50 years of my life.
Both of those things make education an easy target. "Those ivory tower libs think they're smarter than you! But WE were educated by the school of hard knocks! We know what's REAL!" See, the uneducated aren't dumb, they're actually smarter. Remember Rick Santorum bagging on Obama for having two degrees? While he had three? Conservatives only heard the first part.
Conservatives don't have some grandiose plan to make us all dumber. They're just playing their cards to get more votes. And it works! Tell the uneducated they have nothing to fear and are better off, while gutting education to make more voters.