Walz is new. I think they dropped Beshear because they are never going to win KY and even though he's a Southern governor and charismatic, he's younger than Kamala and not particularly well known nationally (yet).
Heavy on governors (expected) and no Mayor Pete (though he's already vetted, so...)
@Cosmichomicide Cooper, Kelly, and Shapiro are the best situated to help secure a swing state.
North Carolina has 16 electoral votes.
Pennsylvania has 19 electoral votes.
Arizona has 11 electoral votes.
You choose Cooper for the proven ability to win NC and the fact he's term limited. Plus totally normie and 16 EC votes.
You choose Kelly if you are focused on gun control, but he has the same "new guy" feel of Vance, albeit with the whole space thing.
You choose Shapiro if you want to throw Penn into chaos (again)? Really no idea - he's perfect where he is.
And yes, I know, Pennsylvania is a battleground and he's popular. But he still has that new governor smell and voters tend to look askance at people who drop the job after a year.