Because Khan has a bigger name than both of em. If there was a fight to be made, Khan would be the A-Side easily over both Thurman and Brook. Brook has only fought in America once and Keith Thurman has never even headlined his own card yet. Khan is higher up on the food chain than those 2 and they arent on his level.
That's pretty much it. Only other guy that is near the floyd/pac level is possibly canelo.