Money.
The De Gaulle was hideously expensive, and the Brits took note. As it is, the Cameron Gvt is now trying to reverse course on it’s decision to go with a “conventional” (cats and wires) launch/recovery setup instead of STOVL ... CATOBAR being much more expensive than they planned for.
Yes. The upfront costs of nuclear power are enormous and the returns are very slow. But it leaves room for future upgrades like rail guns and laser CIWS.