Blame the setup in the postskip arc.
- Fish-Man Island: Fought Dosun
- Dressrosa: Part of the shafted half. No fight.
- Whole Cake Island: No traditional matchups to show how the Yonkou differ from past opponents.
- Wano Country: No one left for Chopper to fight (though he did hold his own against Queen and Perospero).
One of my hottest One Piece takes is I think Chopper could've taken Who's-Who if Dogstorm hadn't shafted Jimbei from the Jack fight. If not, someone inbetween Who's-Who and Sasaki.