Why two British carriers did not justify nuclear propulsion

Was it a mistake not to make the Queen Elizabeth-class carriers nuclear-powered?

No; supporting a unique domestic surface-reactor system for only two ships would have been uneconomical.

Britain would have needed a domestic surface-ship reactor rather than simply adopting an American or French plant. Establishing the entire support and qualification pipeline for only two carriers would have imposed disproportionate cost and complexity. Submarine and surface-ship reactor installations also have materially different operating and support requirements, so an existing submarine plant would not remove that burden. Conventional propulsion was therefore the practical choice for a two-ship carrier class.