Cause I like BP over pyrodex. Pyrodex by itself is corrosive, BP tends to draw moisture out of the air. Different type of rust forms if you can't get your gun totally clean. Rust that forms from BP fouling tends to be like a powder, almost like dust which can be cleaned up very well. Rust that forms from Pyrodex tends to be hard chucks and starts to pit right away. We had a gunsmith who took 2 rifles and fired BP in one, pyrodex in the other. He basicly just wiped out the barrels and took the guns home. 3 days later he started running patches down the barrels. The BP barrel had some rust in it but took only about 15 minutes of cleaning to get it clean and looking good. The Pyrodex had minor pitting and after a half hour still needed more work to get it right. Also Bp works in my flintlock all the time. Pyrodex is iffy at best