Black powder has a slower burning rate than smokeless, which decreases the pressure that forms under the projectile. This same phenomenon may be observed within the range of extruded smokeless powder burning rates. I.e., you can fill a CF case with a slow powder like IMR 4831, and it will still be below the pressure of the same cartridge loaded with a smaller quantity of faster burning power that is maxed out on pressure.
Other factors mentioned above are also valid within the same concept. Slower powder is giving the bullet a gentler push out the barrel; the faster powder wants to spank it out. Likewise, a bullet with a full skirt creates more internal bore friction than a round ball, which increases pressure. Jacketed bullets, more pressure still.
BP will only burn X fast, and after that point the excess powder cannot combust and raise pressures any higher. This is one reason why shotgun reloading data needs to be followed fairly closely.I've tested my BP rifle over a chronograph where I reached a point that velocity would not go up further - the excess powder was simply blowing out the end of the muzzle. I was still far below what the manufacturer listed as a maximum charge for the rifle, so went up some distance confirming this effect.
Other factors mentioned above are also valid within the same concept. Slower powder is giving the bullet a gentler push out the barrel; the faster powder wants to spank it out. Likewise, a bullet with a full skirt creates more internal bore friction than a round ball, which increases pressure. Jacketed bullets, more pressure still.