I believe the main reason is that the big companies don't believe they can make a profit from it. They would have to charge a high price and don't believe the market for them is big enough.

I'm surprised one of the smaller companies hasn't given it a try. One might try it eventually.

YS may be right about the liability issue being part of it, but if it is, I think it's based on perception instead of science. Federal already makes a 15g/cc shell, so the jump to 18 isn't that great as far as safety. The only real danger is that it can scratch your barrel, but proper wads make that unlikely. A tss load is no more likely to blow up in your face than a lead load. At least, that's the way I understand it. There are thousands of people shooting tss now, and I've never heard of an accident that had anything to do with its density or hardness. I could be wrong.