A carrier unlike a sub has lots of resupply ships for food, jet fuel etc, therefore it can be supplied with oil at the same time.
A nuclear powered carrier has to be taken out of service to have the reactor refueled; this can take over a year, with only two carriers this becomes an issue. (Unlike the USA that has lots of carriers, so can be refueling one at a time.)
The calcs show that over the lifetime of the carrier a reactor would cost more then oil.
With a reactor, if it breaks down you got real problems; with oil you can have many separate generators.