The main reason is cost. Not just to buy, develop and install, but to run. QECs do not have large crews, they are built around automation. The RN does not have the personnel to man two nuclear powered ships. It is a technology alien to the surface fleet. A whole new set of skills should have to be added to the equation which means more time and more money.
I don't think the French CVN is a good model for how to run a carrier.
The fact is that the QEC is a UK defence platform. It isn't a fleet carrier of old, it isn't a CVN it isn't a Commando carrier or a CVS.
It can be used in all the above roles and more, but the RN is not going back to the days of Eagle Ark Victorious et al. It is moving forward with two very useful platforms (ships) that will be capable of getting involved in all sorts of operations.
Yes they are going to need dieso and avcat. That's why there are some gentlemen in Korea or somewhere hammering some tin into the shape of the new Tide boats.
It used to be called the Fleet Train, and I think it is one of the few maritime evolutions the USN pioneered. They are still rather good at it in the USMSC.