Well, consider stuff that is far away from the event horizon: if it's far enough away we can just use Newtonian gravity and ask the question there, which is going to be easier.
So the revised question would be: does stuff around large central masses very often form a disk in Newtonian gravity, and if so why?
And the answer is yes, it does: the Solar system is a bunch of stuff orbiting a large central mass, and it's very close to being a disk (with big lumps in, OK). Saturn's rings are an even better case. Galaxies are very often disks (but not quite the same thing, as there is no dominating central mass).
So, why? Well, consider a system where things are orbiting in all sorts of directions. Those things will crash into each other occasionally, and quite often if there are a lot of small things, or gas. The configurations where things crash into each less often will be more stable, because the interactions which change the configuration happen less often. Disks are such a configuration.
Perhaps there are more stable configurations than disks? Well, there's an important constraint on the system: it has to conserve angular momentum. And disks are pretty much the unique configuration which both conserves angular momentum and minimises the interaction rate. So you get disks.
So, far from the event horizon, there will be a disk (the solar system would still be a disk if the sun was a black hole). Closer to the event horizon you need to use GR, but the same considerations turn out to hold, more-or-less.
This argument contains a bunch of hand-waving and omissions: apologies for that. I think it is right in outline.
