This question is asked every year and no one knows definitively why. Here's my best guess based on what I think I know. Years ago I bought a Comp-N-Choke for my Benelli M1. After not getting the results I had hoped for during patterning, I called them and spoke to the owner Charlie Boswell. In that conversation I asked about some of the heavier than lead loads. He basically told me that he couldn't legally recommend it. If I recall correctly it was due to the shot being a stronger alloy than the choke. I am not a physics expert but when two metals contact each other enough, the weaker metal will eventually give. When the barrel is made of softer metal than the shot being used, one has to wonder how many times it can safely be fired until a material failure occurs.

My guess is that corporate lawyers have told the shotshell manufacturers that if an accident ever happens and they are sued, they are probably 99% sure to lose the lawsuit. One lawsuit could wipe out 20 years worth of profits. It doesn't make sense to offer it from that standpoint.