Why was Harris' vote total so low?

Look at these numbers:

2020 election:
Trump 74.2 M
Biden 81.3 M

2024 election:
Trump 72.7 M
Harris 68.1 M (16% less than Biden)

I have been following the campaign here on DK and all the diaries talked about the enthusiasm and eagerness to vote. Tens of millions of door-knocking (thanks snowbored), postcards, phone banking, etc.  Everything looked so rosy.  I could understand that Harris would not quite reach Biden's total , but 13 M less seems inexplicable.

Any ideas about Harris' low number?