I think she has done a fantastic job and would provide a good country to Pence (being from the same region, but a little more pragmatic than he is). But I can think of a number of reasons why not to pick her.
As stated in this article, Obama’s advice to Biden was find the “yin” to his “yang”. That seems to be the guiding star. I think Biden did that for Obama, but I can’t really see how Whitmer does that for Biden.
For Whitmer, she is a popular governor in one of the states Biden needs to take back to win this year. But the whole argument for Biden up to this point is that he will be stronger than Clinton was in 2016 in these states. Primaries aren’t necessarily predictive of strength in the general, but Biden vastly outperformed Clinton in Michigan—and took some of the wind out of Sanders’ sails by taking the majority of white working class folks there.
Another thing is Whitmer, much like Biden, is also pragmatic and more centrist. While that may be good to get some swing voters comfortable, I don’t think independents or moderates are worried about a Biden ticket because he is too liberal. Indeed, it seems Biden recognizes the need to move to the left (with recent proposals to reduce the Medicare age and establish a meaningful precedent of lowering the age that has been constant since the program’s inception and to provide student loan forgiveness for middle class and lower class folks) to unite the party. He needs a candidate that is going to excite his base.
Now, a lot has been said about Kamala Harris, but she seems to be polling well as a VP choice and, as a presidential candidate with as many years in the Senate as Obama had when he ran, can be seen as ready to take on the job on day one. She also represents a demographic in the party that largely came to Biden’s aide when he needed it and whose enthusiastic support he will need to get the Obama coalition back together. Kamala Harris has a rather unique fundraising deal with the DNC that certainly does raise a number of eyebrows.