That depends on how he sees his political future. If he wants to run for office again as a Democrat, he might be crazy to take it. That may also depend on whether he sees himself running outside of WV as a Democrat though. I'll refer you to John Huntsman if you doubt that serving as an appointee of the other party's president harms your chances on the larger stage.
I agree that this is probably about what he sees as his political future. I am skeptical that he can win in 2018 as a Dem in WV. Manchin did virtually nothing to try and save coal jobs and most West Virginians understand that. He openly campaigned for Hillary after she pledged to kill all coal jobs. Not smart.
If Trump campaigns heavily in the state against Manchin, it will have an impact especially if West Virginians see that Trump is working hard to help restore jobs.
I think Manchin would be wise to either convert to a Republican or take this cabinet position if it is offered. JMO.