Chopper isn't fighting Queen for long. There is a reason why Oda used Zoro to indicate they should go to the central floor for help. Chopper will likely end up in a difficult position after showing off his new power-up, saved by Sanji, most likely. In the meantime Chopper would take care of Zoro's injuries assuming Marco gets occupied by King
He has to defeat a strong opponent.
Usopp was useless against Jabura and he had no personal grudge towards him, while Chopper is already fighting against Queen and has many reason to fight him.
And let's not forget Queen attempted to kill Drake and Apoo and Chopper can't move after using his Monster Point.
There is no reason to force Sanji in there, Queen vs Chopper, Drake and Apoo for a bit is perfect the way it is.