Experience as vice president for an incumbent president, and more name recognition across the US.
The presidential nominee needs broad support and recognition, with the VP essentially filling out political gaps. Voters are likely more comfortable with a name they know, leading to greater turnout. It also significantly reduces campaign cost to outcome ratio because the voter base doesn’t first need to be educated about who the candidate is. People have limited attention spans, and you want to focus campaign dollars on the things that will move the needle most. This particular VP pick is not well known across the US, unlike the presidential nominee shortlist before Harris was chosen where some of those contenders overlapped with the VP shortlist.