The .40 caliber was made popular by Dixie Gun Works about 35 years ago. For some reason they selected it for their rifles and kits and since they were the leading source of sporting muzzleloaders, the caliber was accepted for general use, right up to deer hunting. But a .40 ball is not really very big and hunters moved up to bigger calibers, first .45 and then .50 which seems to be the most popular these days, although often with saboted pistol bullets. A lot of traditionalists shoot the .54 and its half-ounce ball.
Now the .40 is not common; it is neither fish nor fowl, small for deer, larger than the traditional .32 or .36 squirrel gun. I don't know if a mass produced .40 is even made any more.