Why does a black hole have much entropy? Is that just an assumption by Bekenstein etc? For example, what is the different states a non-rotating primordial black hole can be in?
 A: If black holes are to satisfy the second law of thermodynamics, then since black holes do not remember what went into them (other than the total mass, angular momentum, and charge), their entropy has to be larger than the maximum entropy of all possible matter distributions that could have formed the black hole.
Some fiddling around on the back of an envelope with photons and the fact that there is an upper bound on their wavelength (lower bound on their energy) for them to be able to go into a black hole, leads to the conclusion that this implies that the entropy of a black hole must scale with (at least) its mass squared. Another thing that scales with its mass squared is the horizon area. It can also be proven that this area is always non decreasing (in classical physics). This lead to the hypothesis that the entropy of a black hole is associated with its area.
Proper confirmation of this requires understanding of the quantum mechanical microstates of a black hole and therefore a theory of quantum gravity. In various proposal for such a theory including string theory and loop quantum gravity, it is actually possible to count the number of states of some very special black holes. These calculations confirm that (at leading order) the entropy of these special black holes scale with their horizon area.
A: Actually there were many debates on it, and some paper also established by Cambridge and Oxford University but actually Nobody really know why the entropy is so big. My view is that black holes manage to excite the quantum-gravitational degrees of freedom, so to really understand them you should work in quantum-gravity. In a such a theory it should be possible to identify a number of microstates proportional to the exponential of the area. For instance, these microstates have been identified in strings and branes in string theory.
