"It is a shame that people chose to punish them for not achieving the impossible"
I think that's one of the reasons. The other is that people voted for them as a protest vote, something they reckoned was "safe" because they (LibDems) would never be a party of government. And then they did the unforgivable - they joined the government. Just being against things isn't a responsible political stand; sometimes you have to be for things and a lot of their voters had forgotten that.