1) As already noted above, today's young college students lean conservative. Besides, registering people to vote and getting them to show up at the polls are two different things. RTV was good at the former, but hapless at the latter.
2) Back in 1992, the average MTV viewer was a college student. But in the intervening years, MTV has moved further and further away from music and into cheap tawdry pop culture pablum, and the average age of its viewers has plummeted. It no longer matters how much support MTV throws to RTV now, because 15-year-olds can't vote.