It comes from cartridges that tend to be older. Like the 9mm, 38 special, and .45 acp. These cartridges are pretty old nowadays and the older firearms were not built to handle the kinds of pressures that we can now produce with are more effective powders these days. So the SAAMI specs on these cartridges are lower than what we can really achieve to assure if someone is using an older firearm, it is still safe. So new firearms are made from more advanced metallurgy and can withstand the higher pressures, the owner's manual will tell you if it can handle +p or even +p+ ammo. Basically they put an extra grain or two of powder in there, crank that pressure up further and gain some velocity. So you don't see +p in new cartridges since they were designed from the ground up with the newer powders that we have. But who knows, in the near future we might make more advances and you might start seeing some +p .40 out there!