Trump chose Pence because he had government experience and his religious conservatism helped reassure the GOP’s religious voters.
Biden choosing Harris:
Biden committed to selecting a woman as his running mate … He noted that his selection would likely be younger than he is
Obama choosing Biden:
Obama recalled that he and his advisers Axelrod and Plouffe wondered if voters would accept a ticket of “two relatively young, inexperienced, and liberal civil rights attorneys” and ultimately Obama felt the contrast between him and Biden was a strength, and that Biden being older than Obama would reassure those voters who were concerned that Obama was too young to be president
In making the selection, Clinton emphasized Gore’s experience with foreign policy and environmental issues.[1] Clinton’s choice of a fellow young southern centrist defied conventional wisdom
This pattern goes through the years. If the presidential candidate is a northerner, they often pick a southerner. If he’s old, he chooses a younger VP candidate. If he’s from a wealthy background, he chooses someone who has a more humble background. If he lacks political experience, he chooses someone who has it. If he lacks international experience, he chooses someone who has it.
All VPs are DEI hires, at least to some extent. It’s accepted that if the two candidates are too similar that the ticket will fail.