Why isn't the Ford class IEP?
The main benefit of IEP is quietness, so that makes you less detectable by subs and for that reason, is most suited for ASW ships. That said the QE class does have IEP. But in my opinion, the question of IEP vs. conventional shaft drives also has the issue of the efficiency of power transmission from the engines to the propeller. Is it really more efficient than conventional shaft driven systems? Converting mechanical energy to electrical energy then back to mechanical energy might entail more loss of energy efficiency than direct from mechanical to mechanical output, even accounting for energy loss due to friction from transmissions. On a carrier or a large ship, using IEP might end up making the ship slower than faster, and you need speed on a carrier to give planes a leg up on lift off. If you are willing to sacrifice some speed for less vulnerability to submarines, you can consider this a welcome trade off. If the designers manage to reduce energy loss and improve drive efficiency to comparable to shaft driven systems, then the speed trade off might be minimal enough to be acceptable in exchange for significant quietness in speed. There is also the question of energy efficiency too whether running IEP or shaft drive uses less energy, therefore improving range, and the question if using hybrid IEP might be possible.