Taranto was one part of a wider Mediterranean operation

Why was HMS Ark Royal not used in the attack on Taranto?

She was supporting the wider operation, including convoy cover and attacks on alternative bases where the Italian fleet might have been.

The attack on Taranto was part of a wider operation rather than an isolated carrier raid. Andrew Cunningham coordinated several purposes at once, including convoy movements through the Mediterranean. Ark Royal operated elsewhere and attacked other naval bases in case the Italian fleet was not concentrated at Taranto. Dispersing the carriers widened the operation instead of withholding strength from the main strike. Cunningham routinely made one fleet movement serve several operational goals.