My assumption from discussions, most specifically on reddit is that many fans are upset that the team traded Isaac Parades, Cody Bellinger and Cam Smith to facilitate some of that (whether or not Bellinger was directly tied to Tucker or just fans perceive it to be that, you can be the judge). Even on the comments on articles here, there is a comparison to Bellinger constantly. And I don't think many people have really checked in on Cam Smith after he had a big series against us (they probably also don't notice Matt Shaw has been a better hitter on the year). I've seen a few times that Cam Smith is essentially the new Lou Brock trade. And despite him not signing a $400m deal yet, I think many people use those types of players to be angry at. And beyond that you have this allure of the big rookie behind him?
When you add it all up, and the Cubs are struggling while the Brewers have surged past, I think Kyle Tucker is a perfect lightening rod for many. I don't agree with it myself, it's frankly ridiculous, but he's a good conduit for ire.