Well for a start here are the votes between the Tories and Labour in the 2015 general election
11,334,576
9,347,304
That's 9.3 million votes so lets get a grip here a moment about Labour being dead.
Here is the 2010 general election results
10,703,654
8,606,517
So both parties increased by roughly 600,00 votes.
There were two parties running an anti-austerity platform, the SNP and Greens
The SNP got
1,454,436
And in 2010 they got
491,386
That's an increase of almost 1 million
The Green party got [*]1,157,613
Then we consider the Lib Dems
2010 -
6,836,248
2015 -
2,415,862
That's a staggering 4.4 million votes lost
Plus there was the UKIP vote which cut into the Labour vote mostly in the North[*]
[*]3,881,099 total votes taken.
And we know the Tories took the most from the Lib Dems as they had a similiar platform.
It looks to me like the Labour party weren't left enough!