1. An energy dampening field was turning off the antimatter. If you believe the TOS torpedoes used antimatter then they would've been disabled once they left the safety of the shielded firing ship.
I should say --- and I should have said earlier and can only plead that it's been a horribly full week --- that I like this observation. (My dear bride does too.) It doesn't just explain why photon torpedoes wouldn't work for other than budget reasons, but does so in a way that explains why nobody would even mention them on-screen. Quite satisfying.