Why did people vote as they did? Ashcroft's post-vote poll
The main 'why' question:
Tory voters were more interested in a 'competent' PM and cabinet, relatively uninterested in promises, and very uninterested in 'the best candidate locally' (well, most Tory candidates are pretty appalling ...). Labour had a lot of 'I've always voted Labour' supporters, and had less voting for a competent leadership. SNP, UKIP and Green voters went for motives, and promises (though many UKIP voters thought they were voting because Farage would make a good PM - a fantasy for them), and Lib Dem voters mainly went for 'best candidate locally' (though they were also doing it as a tactical vote to keep someone else out; that didn't work out well, did it?).
The detailed poll also has comparison of party voters by age, region, socio-economic class, religion (though only 'Christian' v. 'no religion' has enough replies for a meaningful percentage), and a few other characteristics.