Frankly, I believe 9mm is the most popular because both the guns and ammo are cheaper and more readily available than any other. If .40 was significantly cheaper than 9mm, that'd probably be the top dog with the masses. Price is just about everything and most people can't shoot worth a darn anywayCaliber has nothing to do with this though, its stress under fire affecting shot placement. For self defense 9mm is plenty fine, the empirical evidence is overwhelming or it would not be the most popular pistol caliber in the world.