Most of the newspaper media in the UK have always been virulently anti-Labour (Sun headline after the narrow Tory victory of 1992: 'It was the Sun wot won it!). But their campaign went on steroids this time, because they were afraid that a Labour government would restrict some of their powers post-Leveson. Also because some of them HATE the Scots (see below).
In addition, the Scottish Nationalists grew stronger, with a new, strong leader; and with the support both of the disappointed pro-Independence people (a far-from-insignificant 45% voted Yes in the Referendum) and of some people who do not necessarily want independence but are fed up with austerity inflicted by Tories they didn't vote for, and disillusioned by the collaborations with right-wing policies by New Labour and LibDems. It started to appear that Labour and SNP might end up in a position where they could make a deal, and the RW media went to town on how Nicola Sturgeon the SNP leader was 'THE MOST DANGEROUS WOMAN IN BRITAIN' and how her party would manipulate 'Red Ed' to tax the English in favour of the Scots, etc. (It is sometimes underestimated how much some southern English right-wingers have a bigotry against Northerners and Scots, quite as strong as some better-known bigotries.) Ed Miliband ran scared of the RW media, and ruled out any deal with the SNP in advance, which probably hardened the Scottish vote against Labour, while failing to reassure the English people influenced by RW media.
In general, Labour failed to do much campaigning until rather late, and seemed to rely too much on just not being the Tories.
But it is still a bit of a mystery how the polls got it so wrong, and how and why it changed so much at the last minute.
Ugh just ugh.
ETA: The Daily Mail front page just after the election result was headed; 'THIS WAS YOUR VICTORY: HOW MIDDLE ENGLAND ROSE UP TO HUMILIATE THE POLLSTERS AND KEEP OUT RED ED'.
= new reply since forum marked as read