If Labour were really seen as too left wing, then you’d expect them to have lost seats to parties further to the right. As far as that is concerned, Labour lost 9 seats to the Tories. However, the Tories lost 10 seats to Labour, and in addition Labour gained another 12 seats from the Lib Dems, without loss. I make that a net gain of 13. At that level, the ‘too left wing’ narrative fails to stack up. Where Labour did lose seats heavily it was to the SNP, who ran on a ticket at least as left-wing as their own, particularly when it came to questions of equality, rights and the welfare state. There were of course other factors involved in the swing against Labour in Scotland, notably nationalist ones, but on the traditional ‘left-wing’ issues they were outflanked on the left, not the right.
Such evidence as I can find suggests that Labour did less well than expected in England for a number of reasons, some more justified than others: moral panic over a possible alliance with the SNP (based largely the dog-whistle image of Nicola Sturgeon painted in woad, as far as I can tell, not on any actual policies); distrust in their ability to run the economy competently (where competence has to do with technocratic know-how, not allegiance to Thatcher over Keynes); a plethora of images of Miliband eating a bacon sandwich which failed to be counterpoised with any of Cameron abandoning his young daughter in a pub; and a general feeling that although we like the NHS and worship the people who staff it we wish they'd go back to their countries of origin. I’ve seen no evidence at all that Labour’s timid moves towards redressing inequality were seen as too radical by more than a very few people.
I’ve heard Jeremy Corbyn speak on a few occasions now, and I’ve got to say that he’s the only one of the candidates who makes sense, at least to me. For this reason he is, naturally, doomed, but I do think that’s a huge pity.