Its because he just seriously did his job. And apparently thats super rare in todays america. He did politics and good policy without it being a media circus. He didn't do it for attention and visibility. No scandals, no media stunts.
This is what politics shoud be. Not that media circus it is in the US with the goal being to have the most headlines and the most press attention not to make good politics.
My boss and coworker were fangirling when Trump 'won' the debate against biden. I've legit seen them do the same thing with sports talk. Nevermind that Trump didn't actually say anything of substance or truthful during all of that. I just didn't acknowledge them.
To put this into perspective for all you Americans: In my country at least, we don't have "rallies" at all. It's not a thing. We have political debates and news broadcasts regularly where politicians from opposing parties are invited to speak their case, that's it. Of course, parties also have stands in public places where they give out pamphlets and promote their party, but in those places you'll likely find stands from a bunch of parties.
The way you do campaigning in the US is absurd to me.
There's actually a lot of great politicians that most people haven't heard of because they're too boring for our 24/7 news cycle to care about. Walz has just been quietly being an excellent governor to Minnesota, like how Pritzker is doing a pretty good job as governor of Illinois or how Whitmer is doing a pretty good job as governor of Michigan. There's not much of a reason to learn about politicians that don't represent you though, so these kinds of working politicians don't get much attention.