Years... decades ago, effective, mass produced defensive ammunition was, not so good to nonexistent. Certain popular LE cartridges, the .38 super, 357 mag and .45ACP worked pretty well in most configurations, and weren't begging to be "improved". For a number of reasons, 9mm and .38 Spcl on the other hand were found to be lacking.
Early defensive specialty ammo makers like Super Vel and Corbon made custom ammo with high quality bullets pushed to higher than, at the time, normal velocities. As "high tide raises all boats"... everything from .22 and .25ACP to .44 mag benefited from this ammunition "Renaissance". Credit to these early makers... without them to lead the charge, the larger makers may not have felt the pressure to get in the game.
These days, practically every ammo maker makes very good to excellent self-defense ammo, and the Corbon choice is no longer head and shoulders above the rest.