Why Courageous and Glorious became carriers

Why did Britain convert Courageous and Glorious instead of the Admiral-class ships, Nelson or Rodney?

Furious was already a carrier conversion, and Beatty's decision assumed that the fleet would need relatively few carriers.

Furious had already been converted, giving the Royal Navy an immediate precedent for using Courageous and Glorious. The choice is attributed to David Beatty. His judgement assumed that the fleet would not need many carriers. That underestimate helps explain why these hulls were selected instead of the Admiral-class ships, Nelson or Rodney.