I believe Romney, who I supported after the nomination, was the main problem.
1) He failed to gather his base. (he ran a general/centrist campaign from the get-go)
1a) he failed to get a significant number of social conservatives, due to a combination of his Mormon faith and perceived flip-flop/wishy-washy-ness on pro-life/pro-choice.
2) He failed to go for the kill in the 2nd-3rd debates.
He was ahead in the polls after the first debate. Why? He was strong, confident, and bold. He LOOKED like a leader. After he was blindsided by the "moderator" in the 2nd debate, he never recovered that confidence. Unfortunately, many ppl make their decision on such superficial things, not issues.