Tim Kaine, by all accounts, was a reasonably successful governor. He is clearly qualified to step in and become President, if necessary. Picking him won’t cost the Democrats a Senate seat. He gets along well with Hillary. Everyone views him as being sane and affable.
In other words, he appears to be a perfect example of idealism tempered by experience and realism, and a good choice for VP, particularly in light of the quality of the opposition. If the left has a problem with that, well, where are they going to go? Vote for wannabe Mussolini?