I can offer one piece of puzzle: the demographics of registered votes has increased significantly. Take a look at "Age and generation" at
https://www.pewresearch.org/short-reads/2020/10/26/what-the-... .
> The U.S. electorate is aging: 52% of registered voters are ages 50 and older, up from 41% in 1996. This shift has occurred in both partisan coalitions. More than half of Republican and GOP-leaning voters (56%) are ages 50 and older, up from 39% in 1996. And among Democratic and Democratic-leaning voters, half are 50 and older, up from 41% in 1996.
> Another way to consider the aging of the electorate is to look at median age. The median age among all registered voters increased from 44 in 1996 to 50 in 2019. It rose from 43 to 52 among Republican registered voters and from 45 to 49 among Democratic registered voters.
I think that on average, people consider age significant, so they expect their candidate to be around as old as them. I don't think it's the whole picture, but it helps me feel less baffled at least.