Hm, I forgot the Defiant had photons too...
Perhaps quantum torpedos need additional safety protocols, or the Defiant had a different type of quantum torpedo to the Sovereign class (Would explain why they look different and are smaller than the stock on the Sovereigns, and seem to be less powerful) Or both!
[theory] Defiant class ships have less powerful quantum torpedos as the higher yield warheads are found to be unstable when launched from standard torpedo lauchers. When the Sovereign class was in development, the engineering team worked on a safer way of launching the more powerful torpedos; which implements a number of safety protocols to keep the higher yield, and more unstable, warheads in check.
The reason these launchers were not installed on later Defiant class vessels is due to their size, which is a main concern to these small vessels. Second to this is the inability to launch the less advaned photon torpedos, which have a different propulsion system and a strong magnetic field, which can interrupt and destabilise the intricate workings of the stability enhancers. [/theory]
This theory is brought on by multiple observations, Defiants have a different design of quantum torpedo, which appears to be smaller. And the Enterprise E has seperate launchers for photon and quantum torpedos. If I recall warhead weapons are notorious for instability in certain launchng systems.
