Why does black hole has such great gravity?
So for the Earth the gravity at the surface is a formula of the mass and it's radius squared.
Now shrink the Earth to the size of a pea, and the formula is still true but the radius has now fallen to 0.5 inch but the mass has stayed the same.
Now imagine the Sun, 330,000 times as massive of the Earth, squashed to the size of the Earth -- this is about the density of a neutron star.
Now imagine a star 100 times as massive as our Sun and squashed down to -- well nothing. Infinite density but zero size -- this is a black hole. An object with the mass of 100 suns but with zero radius. So you end up with an object with so much mass and zero or at least negligible radius that all that mass is compressed into such a small area that's it's gravity is so great that not even light can escape it.
I'm an avid amateur and not a scientist. But I think Einstein's idea of space-time will help. Einstein suggests that mass causes a warping of space-time itself. This is why the gravity of the sun, for example, will appear to 'bend' the path of light from stars behind the sun. This has been observed in several different settings. The larger the mass, the stronger the warping. A black hole is so massive, and its gravity is so strong, that the warp in space-time is extreme. If you were a photon within the event horizon of black hole, it would still seem to you that you are traveling in a straight line, but in fact it would be a straight line through an unimaginably distorted space. You would buzz on and on, never finding a path that takes you back out through the event horizon.