Because they don't have many hardline supporters like Labour or the Tories. Because people are not used to coalition governments and ludicrously expected them to be able to deliver their entire manifesto. Because it has suited the Tory party and their friends in the media to make the Lib Dems the whipping boys of government to take the heat off their own performance.
That's definitely part of it, but I think there's another part as well. I think a large number of LD supporters 5 years ago were of the view that they were a left leaning, "nice" party, who cared about the environment and the young and were pro europe and kind of open in outlook.
Then they got into bed with the Tories and bad things happened. Bad things that LD voters most certainly didn't want to happen. People felt betrayed, basically, and the tuition fees thing has been such a weight on them, because it's like a beacon of that betrayal.
Even though by and large its now seen that tuition fees were a good thing ...the reforms have not just helped universities, but they have done more for the poorest students ...and the evidence bears it out.
Now there's a claim that needs substantiating!