Your Steam example is specifically Valve characters, which makes sense as that’s what people go to Steam for, mostly.
But Itch isn’t a game developer, so you’re left with a mishmash of whatever different trends are on at any time, which as of now is horror games. So.. I guess Baldi? Then again, Baldi is on Steam, too, IIRC.
To be honest, I don’t think there is an answer. No Itch game has ever really been a “killer app” for the platform.