The problem is that all our MPs signed a personal pledge to vote against any rise in tuition fees, and then most of them voted for it.
This really is it. Especially where the student vote is concerned the Lib Dems are perceived as having fought a single issue campaign, and then done a volte face on the single issue.

This applies even outside the student vote, given the selling point of new politics/personal trust/etc. (I don't think the Laws expenses case coming so soon helped either - the supposed compassionate reasons didn't fly, because the public knows that said reasons don't count when someone is on benefits, so it came across as the same old "one rule for us".