Why one Ark Royal was not simply multiplied

Why was there only one Ark Royal rather than several?

Treaty constraints and differing carrier roles led the Royal Navy toward the Illustrious-class fleet carriers instead.

Ark Royal entered a fleet that already included Courageous, Glorious, and Furious in strike-carrier roles. The Royal Navy then needed a better-protected carrier suited to operating with the fleet. Treaty limits forced each new hull to fill a specific gap rather than simply repeat an attractive design. That requirement produced the Illustrious-class programme instead of multiple Ark Royals.