In addition to the "got blame but no credit" answer that others have offered I would also suggest that they have previously been the preferred "none of the above" response for people who wanted to express dissatisfaction with either Tories or Labour depending on who happened to be the more popular party in their riding; by being somewhat between they two major parties they picked up a lot of votes from dissatisfied by somewhat engaged voters. Once they were a part of government they were a part of 'the establishment' and so the protest votes went ukip or green depending on the voter's base political preference.
Speaking personally, I voted lib dem in 2010 on the basis that they were a party broadly of the left, but distinct from the Labour party of the time in a number of positive ways. To then form a coalition with the party of the right and support a number of measures that I think were quite regressive did not match with the party I thought I voted for.
Of course after seeing 10 days of government without the lib dems to moderate I'm starting to re-evaluate how positive an influence they likely were...
