Why did only six aircraft in the first wave carry torpedoes?

Why did only six aircraft in the twelve-aircraft first wave carry torpedoes?

The harbour had too little room for all twelve aircraft to make torpedo runs and tight escape turns among ships and balloon cables.

The twelve-aircraft first wave assigned six Swordfish to torpedo attacks. Two others each carried flares and a pair of 250-pound bombs, while four carried six 250-pound bombs apiece. Torpedo aircraft needed room to line up, release, and turn away inside the harbour. Sending every aircraft through those manoeuvres together would have crowded the attack among ships and balloon cables.