Maybe it's the time to go back to how things were when the US was founded. We have a vote and top two are President and VP nominees. Keeps the party aligned to what we actually care about and who we think is best, vs who the nominee or party thinks might be best.
More democratic this way
And if it's just for one party vs both, what would that be bad?
I get why it was changed for VP for the dem vs Republican nominee. But why wouldn't we want, as a party, better representation on who we voted for.