He’s a sitting senator of a red leaning state who won because of who he is. He has to vacate to run for VP, which is bad for keeping that seat. Losing the senate easily hamstrings any efforts a President Harris would bring, especially with GOP being the way they are.

Granted, that can be a problem for all swing state guys. Just isn’t as terrible for governors (although that can screw with other things, like voting rights in a given state).