Cost and different ballistics. Not the same as lead. Bismuth is not easy to find in a lot of places. Why not cut your lower leg off and get a prosthetic leg instead of taking care of an infection after all the prosthetic will let you kind of walk Bismuth and ITX are a lead shot replacement, a prosthetic substitute, with real draw backs. Bismuth is a lesser replacement. A step down in effectiveness at a much higher cost, $180/7 pounds verses $40/25. Lead is 100% double gun safe. I won't use ITX shot in a classic double and it also is not cheap.
Did you know that the braintrust in the US military had decided planes would not need machine guns in the future. They tried it for a short while and found out rockets could not replace the machine gun. You had to expend several 100k rockets to do what a few hundred rounds of 50 cal could do. So they went back to putting machine guns on planes. Progress is not always progress in the real sense.
