Mykeal nailed it.
Pyrodex and other BP substitutes were developed to get around onerous shipping and handling regulations that handicap the availability of Holy Black, period.
Regional zoning and fire laws also dictate what you can have stocked in your retail establishment.
If you're in a place where you can get the real thing, you're fortunate. If not, then you have to resort to the substitutes.
A lot of shooters won't allow BP substitutes in their guns, myself included.
I drive about 160 miles round-trip to get my Goex FFg and Cartridge. You can also mail-order the stuff, but you'll pay the HazMat fees.
"Bother", said Pooh, as he chambered another round...