UKIP won 12.6% of the vote, as much as the Scottish Nationalist Party and Liberal Democrats combined. They failed to pick up constituencies both because their votes were spread over a larger area and because of the corrupt way taht districts were created. Also, the media campaign against UKIP and the fear of the Labour and Celtic false nationalists forming a government pushed many to vote for the wet Tories, who would not otherwise have done so.
http://www.bbc.com/news/election/2015/results