The sentiment that, "OMG he's a RURAL democrat!!!!!!!! He'll TOTALLY SWEEP THE SOUTH/GOP DOMINANT PLACES NOW" is, as someone who studies American politics, so annoying to see time and time again. I mean maybe nominating Manchin starts a trend, but some of y'all are really overestimating general party entrenchment in some of these regions. And overestimating how much an individuals actual policy positions matter. Many times regardless of personal position, voters see the big ticket positions of their party first.
And let alone that for these candidates to even get the nomination, they have to go insular towards their bases during the primary, which will hinder their center appeal even if only a little.
Anyway, the reasons why just nominating a candidate that seems to fit a mold isn't enough to flip a region are manyfold.