TTT,
Cameron's biographer:
// Electoral success alone holds the modern Conservative Party together. Cameron’s failure to win an outright victory in the 2010 general election weakened him severely.
The Conservative Party in the country, and a hardcore group of MPs, have never forgiven him for not winning against a heavily discredited Labour prime minister at a time of economic crisis brought on by Labour’s policies in power. The one thing that a blue- blooded Conservative understands and respects is winning. That’s why they loved Thatcher. She won three general elections for them.
Entering a coalition with the despised Liberal Democrats compounded his error in their eyes. His critics wanted him to head a minority government and win a general election a few months later, as Harold Wilson did in 1974. //
by Anthony Sheldon official biographer.
http://www.standard.co.uk/comment/anthony-seldon-david-cameron-could-be-scuppered-by-his-own-party-9258839.html
Conservatives want a Conservative Government NOT a Coalition one. The want a Conservative Prime Minister not a Conservative leader of a Coalition.
That is why they will not tolerate a failure to win in 2015.