It's a "brand recognition" kind of thing.

I think it's because that's the more distinctive part of her name. "President Harris" sounds kind of generic, like the fictional president from an action movie.

It may start to constitute a pattern that the same was true of Hillary Clinton, though in that case it was likely that just saying "Clinton" might cause confusion with Bill.

Also Bernie Sanders is mostly referred to by his first name, so…

Trump™ Kamala™

They should get married. Then it would be Kamala Trump