It’s basically 50/50 with either Biden or Harris at the top of the ticket. Everything is in the margin of error, and polling has been notoriously inaccurate with Trump on the ballot.
So you have to basically ignore the simple Trump vs. Biden or Trump vs. Harris or (Trump vs. anyone else you can think of) numbers because it’s pretty much unknown. But the data says a majority (even an majority of Democrats) want someone other than Biden on the ballot at election. BTW a majority of voters also want someone other than Trump on the ballot too.
There’s also some data to suggest Trump is making some inroads with young male Black and Hispanic voters. Harris will negate a significant amount of that immediately and potentially even more when the GOP can’t resist blowing their racist dog whistles and show voters who they really are.
So it’s kinda about looking at the data, but I think a large part of it is simple campaign facts. In times past a Presidential candidate would do two (sometimes three) rallies in two different states per day. And do interviews while traveling between campaign events. Trump isn’t capable of that pace. Biden most certainly isn’t capable of that pace. Harris can do that. We really haven’t seen a 100% balls to the wall presidential campaign in a while because it’s been two old guys in the last election and in this one… until now.
Remember Biden also had to do the job of being President of the country while also campaigning. That’s a lot of work for even someone young, and Biden is so very old. Sure Harris is VP, but that’s mostly just getting some briefings (too keep up on events in case she might need to take over as Prez) and breaking ties in the Senate (which probably won’t be needed between now and election day). She can devote almost all of her time to campaigning while Biden couldn’t.