Why do almost all chicken and turkey sausages have pork casings??

It's amazing how difficult it is to find pork-free sausage! Many folks do not eat pork for religious reasons (both Jews and Moslems) and would happily eat poultry sausages if they were pork-free. Indeed, I would think that the greatest market for poultry sausages is with folks who do not want to eat pork. I think that pork-free sausage casings are readily available, and don't understand why more producers of chicken or turkey sausages do not use them!