To be honest, as much as I like Kamala, I would have picked Gretchen Whitmer. I would only have pledged to pick a woman.
Why? She's a polarizing figure, she hasn't even served 2 years as governor at that time (ladder climbing), and her accent would set back the ticket.
She's only polarizing among MAGA cultists, which Harris is as well. From what I've read, Joe Biden had most "personal chemistry" with Whitmer, who seems like a real cool lady. Other presidential or vice presidential candidates had the same or less experience, so that's not really that much of a factor here. Also remember that Whitmer served in the state legislature for a long time and was Democratic leader. Furthermore, she was from a critical region and won her election in a Trump state by ten points.
If Whitmer ever decides to run for president once Joe Biden is done, she'll definitely be my first choice. Maybe unless T-Mac ever runs.