A black hole has no more gravity than the same amount of matter in any other form.
But remember that the gravitational forces are stronger as you get closer to the center of the body. The mass of a black hole is packed into such a small size (theoretically zero !) that you can get very close to its center. THAT'S where its gravity is hugely strong.
Answer:
Because matter has been squeezed into a tiny space.
Step-by-step explanation:
According to NASA, this can happen when a star is dying.
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