Material can come in towards the black hole from any angle. However, because matter can't just pass through other matter, matter travelling in the wrong directions will collide more frequently than matter travelling in the right direction. Because of the conservation of angular momentum, the "right" direction will depend on the average angular momentum of everything in the cloud that is collapsing into the disk.
You might be interested to know that this is the same reason all the planets in our solar system orbit in the same disk: all the matter that is now in our solar system was originally a very thin cloud of gas with a small amount of overall angular momentum. As gravity drew it together it flattended out into a disk and eventually the clumps became planets (and the sun in the middle.)
