IMNSHO it was uncertainty, the fragmented choices, fear of quadruple dip recession (think China right now) and ineffectual politicians.
The public, en mass, did what we all do faced with choice we can't resolve. Go with the familiar. In this case the incumbent. If we were flushed with money, generally, we would be more adventurous.

Anyway, as trivial as it may seem, they picked the wrong Miliband. The pretty one would have fared better. Think Michael Foot. When did he win an election? Sorry to disappoint all those level-headed posters here, pretty wins votes and it may only be a few (in your eyes), but every vote you win, doubles the delta!

And look who we heading towards getting as head of opposition. A latter day Michael Foot.

History repeats itself, it has to, nobody is listening.