With all due respect, you're not from a purple, middle America state. A Dem will have to win some of FL, NC, VA, PA, OH, IA, WI and MI to win next time around. These are the middle America places Hillary under performed in last time, and white folks in most of those states went with Trump. Al nearly lost his 1st run when his past was almost successfully used against him in a pretty blue state. He won fairly easily the 2nd time around after Minnesotans became more comfortable with him. A nationwide contest would be more like his 1st senate run as he'd be reintroduced to the entire country, and he'd certainly face the same tactics Coleman used against him the 1st time. When you read a transcript of a joke without the assistance of context, facial expression and inflection, you can entirely miss the joke and even be offended by the content. Add to that, he went to Harvard. Not saying he'd definitively lose, but he'd have to spend a lot of time explaining away past jokes and statements. I think he realizes this and wants no part of the level of scrutiny he'd get in a national run.
All of that said, I wouldn't rule out him being a potential veep. As Dan Quayle can attest, a veep can have lots of issues, and it won't matter much. Al is a good messenger who would be of assistance on the Sunday a.m. shows.