He is a good man who would have been a much better president than our current sad-sack whiner. That said, his backstory makes him hard to elect.
Romney endorsing public health insurance as governor of MA robs him of the issue of Obamacare, perhaps the single most irritating aspect of the incumbent's tenure. His track record in industry as a cost-cutter turned off white blue-collar voters, an important GOP constituency.
Had Mitt been the nominee against Hillary, the election would have turned into the Retread Bowl - old news vs. old news. My hope is the GOP nominates someone younger who nevertheless has current executive (as opposed to Senatorial) chops, like a successful sitting governor. Examples include Walker, Kasich, and Christie.