I think there are psychological reasons that squares typically bet the Over (if that stereotype is actually true).
Squares tend to be risk-averse. When they bet, they're trying not to lose (instead of trying to win).
Sharps don't care about risk, they're simply looking for the best value. Sharps are trying to win.
When you bet the Over, you can win your bet in the 3rd quarter, 6th inning, etc. Squares are drawn towards that sense of security. Once the game goes Over, you can sit back and count your money!
But when you bet the Under, your money is always @ risk. You could have 14 points in the final minute of a basketball/football game, you could have 5 runs in the 9th inning, etc.