Usyk just doesn't have the same buzz at heavyweight that Joshua does- a big fan base in a wealthy country, English speaking media etc.
To the casuals also- if they see Joshua is about the same size & has beat some good fighters, it looks like a good matchup.
Going to be hard to get the same hype going about Fury beating up a Ukrainian cruiserweight 6 inches shorter than him.
I think it would be an interesting fight, but Joshua getting his belts back & then Tyson beating him to claim undisputed is going to be more recognised than Fury beating up a cruiserweight (no matter how talented), to most people.
I guess all that can change if Usyk beats Joshua- but he'd have to beat a few other heavyweights too before it looks like a good win, rather than a "should win" for Fury, imo.