Why cancelling Taranto only postpones the fleet attack

How much harder is the Mediterranean war if the Taranto raid never happens?

Much harder at first, but Britain would probably attack the concentrated Italian fleet in port at a later date.

The historical raid bought the Royal Navy time by reducing the Italian battle fleet at a critical point. Removing it makes Britain's Mediterranean task substantially more difficult. The Italian habit of concentrating major ships in one harbour would nevertheless recreate the opportunity. A similar British attack would probably occur later rather than leaving the fleet untouched permanently.