F-35C has a slower roll rate and other performance compromises that make it a noticeably worse air-air fighter than F-35A.
With many NATO nations relying on F-35A to be their sole fighter, that is a very important issue.
F-35C is significantly more expensive than F-35A... and no amount of production-number-shifting can cure that.
This is because of the differing materials and so on used in the carrier version.