Actually it only matters in close elections. Electors can’t vote for both. So I’m guessing they chose Bush. And some no name for VP.
In a close election Trump could get stuck with the Democrat VP nominee if he chooses Rubio.
That would be a terrible outcome.
Can you imagine the Demoncrat response if they knew single bullet could change the White House from R to D?
They would fire up their crazies to take any chance they had at Trump.