2. They tell us that he ran a lousy campaign.
3. They tell us he was the wrong candidate.
All true and I'd add 4. They tell us that the lack of enthusiasm for Romney hobbled his campaign, denying it the ground troops necessary for an effective GOTV operation. That was what I was afraid of when I saw not just the apathy of most regular Republicans toward the candidate but the open hatred for Romney by a substantial minority of the conservative movement. The people (like me, for example) who grudgingly pulled the lever for Mitt didn't have the heart to volunteer, donate money or do anything more than the bare minimum.
The same seemed to be true for the Democratic base, but as mentioned above he started far enough ahead that he could afford the losses. Mitt couldn't.