BP is a low grade explosive, and they make it difficult to obtain to keep it out of the hands of terrorists, and other irresponsible people. That is a joke, of course, since criminals steal most of what they want or need to do their work. Break ins to Construction sites to steal explosives occur with wome regularity, to the point where many companies are now using a liquid mix, and no longer use any form of dynamite. There are also recipes for making your own C4. And, gasoline is not regulated at all, and was the explosive used to blow up the Murrah building in Oklahoma City.
So, while I can give you the reason " why ", Don't for a minute think I believe this is anything more than a " feel good " measure by government, and a further attempt to drive civilians away from owning and shooting firearms as recreation, or self defense. Congress merely looks the other way( that includes the so-called Pro-gun members) while the bureaucrats write these regulations and then enforce them as law. The fact that not one Congressman or Senator has introduced legislation to repeal these restrictions on Black Powder speaks volumes about what they really think about gun owners. The same can be said about the Lautenberg Amendment, that imposes a lifetime disability( prohibition) to owning guns for anyone even accused of domestic violence.