Engines.
Yes the Spey story is a good one. For RR. On the strength of equipping the ARK with its single squadron of F4Ks, the
entire UK F4 buy were equipped with Speys. Which cost about twice what the standard F4J was then going for. Amazingingly enough, sufficient to have procured three new carriers, (with decent sized decks) I believe. That the Spey Phantoms were pretty duff can be seen from the fact that the Air Force got rid of 'em with almost indecent haste, (to replace 'em with the lovely F3, of course) And the Luftwaffe has only just retired the last of its ancient F4s, but even these were not too shabby at the end, with AMRAAM and decent radar.
When looking at taking F-35C and putting it on CVF, we could consider this piece of history....
Well yes, but the CVFs are big boats, over 60K tons. If the frogs can operate RAFALE off the C de Gaulle, about 45K tons, are you telling me that we can only operate a STOVL off these big, brand new ships?
Plus of course by going ski jump we then have to make do with rotary wing early warning aircraft, the frenchies have E2C, and so it goes on...