UKIP probably gave the victory to the Tories. A lot of people probably secretly like the idea of UKIP but knew it was a wasted vote, thus the next nearest was Conservative. The promise of a referendum likely helped too.
UKIP had no obvious plans for anything so was never credible.
Meanwhile Labour had a leader that wasn't fit to lead, and a shadow Chancellor that was repeatedly made to look stupid. Saying 'cost of living crisis' over and over didn't work, and the mansion tax wasn't going to raise enough money to be spent on just about everything - as seemed to be the case. How many times was this mentioned as some sort of unlimited pot of money?
What's more, many people probably realised that the 'nasty' rich people expected to pay for a huge spending spree under Labour likely included them. Basically anyone with a job above minimum wage would be considered fair game. There just aren't enough billionaires to fund a huge increase in spending.
Oh, and all the attempts on social media to get Labour in (including a last minute vote of approval from Russell Brand) probably failed to realise that a lot of young people can't actually be bothered to vote. Tweeting #gettoriesout and liking pages isn't actually enough to change Government. Facebook and Twitter failed miserably.