I wouldn't even go so far as to say it's that they "like" Harris.
She seems good. And Walz was a truly inspired pick for running mate in the current political climate of the United States. But I think a LOT of this excitement comes from just not being old as fuck.
The size and immediacy of the reaction to her candidacy tells me that there's a sense of almost cathartic relief that there's actually someone in the game that can actually still function AS the president. I'm sure Harris and her personality have something to do with it as well; but at least some of the "wave" as they call it, would have existed for literally anyone on the ticket who could dress themselves compared to the two old geezers they had to choose from before.
This is just my take as a Canadian watching, suddenly interested, from the North.