Manchin is a career politician. He's been in the Senate since 2010, but he was governor and SecState of WV before that.
While he somewhat has a reputation of being a "moderate Democrat" leaning often toward conservatism, he's riding on the coattails of Robert Byrd and the "Democrat" moniker that goes with that.
Manchin understands his reputation and is not afraid to put his stuff out there. Why? To stay in front of the cameras, of course. At the heart of everything, Manchin is a career politician.
And while Manchin may enjoy the "lone wolf" Democrat/Blue Dog-type reputation, we must always remember for every Manchin, the Republicans have a Mitt Romney, Susan Collins, Lisa Murkowski, and Ben Sasse. So maybe Manchin's statement isn't quite the blockbuster that he'd like us to think it is.