Very good question. The answer that I have always been told is that being a male porn star is a lot more difficult than you would think. They have to do a 3 hour porn shoot with a hot chick and walk the fine line between being totally limp and premature ejaculation. And they probably have a small pool of guys to work with - how many guys are willing to do it with the stigma attached to it? And out of those guys, how many can perform as I stated above. And out of those guys how many of them are hung?
Well let's see:
out of 7,000,000,000 people on earth,
roughly 2,000,000,000 are considered "first world".
Only half, or 1,000,000,000 of those are male.
If only 1 out of 1000 are willing to try porn, your down to 1,000,000.
If only 1 out of 50 are able to perform, your down to 20,000
If only 1 out of 20 are reasonably hung, you are down to 1000.
Out of that measly world pool of 1000, how many of them will continue to perform over time, are willing to stick with it when the money isn't all that great and don't get all messed up with drugs, etc, etc??
That does probably explain why its the same 10-20 guys in porn. Just my thoughts.