Because these are, essentially, apples and oranges. Einstein's theory of general relativity, and his field equation for gravity that derives from general relativity, describes the shape of space-time subject to the influence of the matter and energy contained within it. The famous 'black hole' solutions describe what happens to space-time near gravitational singularities contained within space-time. In other words, they describe what happen to the geometry of space as a star collapses inside its own event horizon...the mathematical surface surrounding the star within which light cannot escape. The Big Bang, however, is a phenomenon in which space-time was created so that there is no exterior space-time in which to define an event horizon!
Because the universe has a finite age, surrounding every observer there is a horizon to the observable universe beyond which we may not receive information. Our horizon is at a distance of 15-20 billion light years, and any gravitational influences farther away than this will have no effect upon us today. In billions of years to come, this horizon will continue to expand outwards in space until it eventually engulfs all of the space contained in our universe. This horizon is similar to the event horizon of a black hole only in that it represents an information limit beyond which we cannot know what is going on. It does not, however, hide from us a singularity in space-time such as the one inside a black hole.
Some people have noted the parallel between the 'singularity' at the Big Bang out of which the universe and space-time emerged, and the singularity within a black hole. Since matter and energy emerged from the Big Bang singularity, it bares some resemblance to the so-called 'white hole' solutions in general relativity, which are time-reversed versions of black holes in which matter/energy are continuously being emitted rather than gravitationally attracted. In the Big Bang situation, this parallel would require that we live inside the event horizon of the cosmological singularity. The problem is, again, one of exterior spaces. Einstein's field equation describing these white hole solutions still requires the pre-existence of an exterior space-time within which this white hole singularity occurs. The Big Bang solution does not presuppose the existence of an exterior space-time in which the entire space-time that makes up our universe came into existence. The solution itself merely describes the unfolding of space-time from this incomprehensible Cosmological Singularity which is, mathematically, of a different class than any black hole or white hole singularity.