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I am guessing this has something to do with density.
I would assume that a massive star that has its core collapsing would be a prime candidate for having its core turn into a black-hole. If the core then turned into a black-hole, it would follow that subsequent mass would rapidly pass its event horizon and feed the black hole resulting in a rapidly expanding event horizon.
So why does a supernova occur at all? Why does the star not simply completely collapse into a black hole with no explosion when a star's core collapses?