For a couple of reasons....Conicals or Minis came out a few years before the Civil War...They were made to be able to load the gun quickly, so they are made just smaller than bore size and have a hollow base...Upon firing, this base flares and that's what engages the rifling...For this reason the rifling is shallower than a deep groved round ball rifle...Also it takes a faster twist to stabilize a conical than a round ball, due to the conical being longer....

My 54 caliber flintlock has a Ed Rayle barrel, cut rifling, .012 inches deep and a twist of 1-72...Your typical conical rifle will have button rifling .006 deep and a twist of 1-48 or so...A conical would have a hard time filling the deeper rifling of a round ball barrel....

Make sense???