Why only one interwar Ark Royal was built

Why was only one Ark Royal-class aircraft carrier built?

Ark Royal completed the immediate strike-carrier requirement, so the next priority was the fleet-support role, followed by replacements for Courageous and Glorious.

Ark Royal joined Courageous and Glorious to give the Royal Navy three ships for the strike-carrier role. That met the immediate requirement when the next carriers were being ordered. The Illustrious class addressed the separate fleet-support role. Implacable and Indefatigable could then replace the older Courageous and Glorious in the strike role.