Cameron hired Anita Dunn to advise his campaign.
Nothing else need be said.
And yet, Cameron lost no votes to his right. None, zip, zippo, nada. There's not a party to the right of the Tories that have even a single seat in Parliament. The fact of the matter is the Conservatives have become a permanent minority party in the UK. No one voted for Labour or the LibDems because Cameron had Anita Dunn advising him.
Cameron tried to get people who were center-left to vote for him because there just aren't enough people center-left to give him a majority. He couldn't get it done. It's not that surprising.