Because it's hard enough to sell 40 shirts!
Actually these shirts I've found tend to be bigger than most -- a medium was bigger than my XL shirt from thinkgeek... though they may shrink some, but I don't think they'll shrink THAT much. At the very least, I don't think there will be a problem with them being too small.
I'm still trying to get the next order in. Problem is, the place I got them done at doesn't do things for individuals, but the guy at Atlantis Gaming Center knows the owner there, so he managed to get it done for me last time. He's been pretty busy with the move and all recently, though.