Apparently Jimbo's position as a milestone for Zoro/Sanji and now a ceiling for Franky/Robin is too much complexity.
Jimbo has reach his peak. Any bounty increases will reflect, more than anything, the usual "crew default increase".
Had it not been the case, Jimbo would have been matched against Jack, a Calamity.
Yet he had to fight a Tobi Roppo, just like Franky and Robin.
Jimbo's not a fraud.
It's just that, in the very arc Oda shoves down your throats an obscene Sanji "left wing/hand" of the Pirate King parallel, He couldn't let Sanji of all people be the hypetool here.
It's the endgame.
Jimbo serves as the hypetool even better than Sanji, because Jimbo's power is notorious around the world.

