Successful brainwashing is what did it. The RINO establishment has been drilling this bit of fantasy into the mushy heads of the party faithful for quite a while, and now many believe it. The last Republican Presidential nominee who ran on a strong conservative platform (emphasizing small gov't) was Ronald Reagan, and he annihilated his Democrat competition in two straight elections:
see 20
And the year leading up to RR's reelection, (the largest victory in Presidential election history), his book Abortion and the Conscience of the Nation came out.
There were folks who had advised him that maybe this wasn't such a good idea before an election.
RR thought otherwise.
You're right, it is because of the media and their endless stream of blather. They want someone of either party who is soft on social issues, so they peddle the line that the only way to beat a liberal Dem is with a liberal Republican. I'm just surprised that so many self-proclaimed conservatives are biting.