Do you mean "cap" as in percussion cap? iirc, they were invented in the late 18th or early 19th century. Cartridges predate percussion caps. It's just that they were paper cartridges, which included the correct powder charge, the ball, and the wadding.
Also, black powder is hygroscopic. It will absorb water from the air and stop being sufficiently flammable to work as a propellant.