The temperature involved for this is well above that needed for production of electron-positron pairs, so one may ask why that doesn't happen. I think it's because the cores of such stars are dense enough the electrons are degenerate, and there simply isn't "room" for new electrons to be added at an energy that would make pair production possible.
BTW, I don't think largeness is needed for photodisintegration to occur; this should happen even in garden variety type-II SN.
https://en.wikipedia.org/wiki/Photodisintegration#Hypernovae