Cost, both initial and maintenance. Though I'd hope the French Navy avoids "fitted for, but not with, 3 catapults" like the incoming nuke it is.If the third 'cat' is possible, why not?
That's running into manning issues more than buying the extra steel to make a 100kton ship instead of a 75kton ship. A Coral Sea-sized angled flight deck really only has space for 3x catapults, there's not enough beam to allow an inboard waist cat to clear the landing pattern. But there is space for 2 cats in the bow and 1 at the waist, and for all 3 of those cats to clear the landing pattern.However I'd say it's more about what the French Navy can "afford" rather than "want". I'm sure they'd love to have 4 catapults on a Gerald Ford sized carrier but without negatively impacting the rest of the fleet.