Because they have the same view Crab outlined. Lib's are seen as traitors to the left who let the tories get in, and they should just drop their idealism, join labour and beat the tories. It's as if the conservatives are a nebulous enemy you hate at a distance, whereas the Lib's are your next door neighbours who piss you off more because they live right next to you. The conservatives are evil by default in this world view - whereas the Lib's are people who enable evil by not joining labour. for some people that makes them worse.
(Note, not my own views!)
It ignores the very real policy differences between the two parties, and takes it for granted that all lob voters would vote labour if there were no Lib's. It also misses the point in that labour could fight for those lib voters if they wanted to.
Not my views, either. It's just very understandable because it is probably true - I think it is very difficult to argue that more liberal things do not get passed in the world where the Liberal Democrats cease to exist as a party; in the same way it is probably true that more leftwing things get passed if the American Greens just didn't exist as a party (2000...). Now, I don't take this very mechanical view towards these things. The democratic system does not work at the point that the electorate are simply required to stamp off on one of two choices, both of which are sub-par, in an attempt to stave off whichever one they feel is slightly worse. The right of the voter to chose as they may is paramount to democracy, it takes place before you can make any other suppositions; questioning it leads to dark places.
But regardless, it is incredibly easy for Labour to see Liberal Democrat seats as: stolen seats. Conservative seats, well, you have to fight for those, but Liberal Democrat seats feel like they should almost rightfully be Labour's and have been unfairly whisked away to be put to very little use. It's true that there are policy differences between the Liberal Democrats, and it is true not every Liberal Democrat voter would vote Labour if the Liberal Democrats did not exist. But it's also true that the Labour party is the second choice of the clear majority of Liberal Democratic voters. And the coalition just made this an extra kick in the teeth, because those seats were, for the first time in a long time, not just wasted - they were actively put towards Conservative goals and values. It was a betrayal, of sorts. The Liberal Democrats took our votes, and they built a Conservative government with them. It's not at all unsurprising many Labour supporters were happily dancing a jig on Nick Clegg's grave.
That'll fade, over time, as memories of the coalition go. Most of us here are too young to remember, but in the late '90s, relations between Labour and the Liberal Democrats were excellent - they nearly formed a coalition despite Labour already being a majority party. But in the present era certainly, when Liberal Democrats talk about a progressive coalition, the first reaction for much of Labour is to say: "progessive?
you? remind me, which party was in government during the greatest weakening of the state since the second world war? which party was in government when child benefits were slashed, the education budget fell in real terms, the NHS was made less accountable?" That's not going to be overcome for a while yet.