The bedroom tax hasnt gone down as badly as some are claiming.
It was a disastrous policy for Labour to use front and centre.
The basis for it as a policy from the Tories is pure spite: the sums of money involved are trivial. However, the Tories may be heartless scum, but unfortunately for Labour they are politically astute heartless scum, and they realised that (a) the myth of the benefit scrounger being given a jacuzzi on the rates actually is strongest, and plays best, amongst the working poor and not-quite-poor and that (b) the bedroom tax isn't going to be a major issue amongst people who don't live in social housing anyway. So it's spiteful social policy, pointless financial policy, smart political policy.
The only people that endless banging on about the iniquity of the bedroom tax appeals to is a portion of Labour's own base, who either already vote Labour or don't vote. Meanwhile it alienates another portion of their own base, people who see themselves as hardworking and ill-treated (ie, the people Emily Thornbury sneers at, because they don't even buy the right sort of ciabatta). It's a gift to the Tories and to UKIP, as it allows them to paint Labour as the party of the workshy. Yes, I know that's both ludicrous and nasty, but the genius of the evil Tories is that they fight the election that's happening, rather than Labour's habit of fighting the election they wished was happening.
Chuck in Labour's failure to produce a credible policy on immigration because they were more bothered about the reaction over Islington dinner tables than in actual marginal constituencies and why would anyone who didn't vote for Labour in 2010 vote for them now? And they didn't. I'd vote Labour under any circumstances (hell, I voted Labour in 1983, unilateral disarmament, leaving the EU, leaving NATO, nationalising my sock drawer and everything) but I can see why they lost this time. Labour's campaign was incompetent. They were hoping the Tories would screw up, and the Tories very rarely do that.
In my constituency, Labour slightly increased its vote (yay!) and a very silly Tory candidate funded by her rich husband rather in the manner of a cupcake business who loves local schools so much her own children are at boarding school 100 miles away got soundly beaten. On a decent turnout, too.