The modern in-line muzzle loader/black powder rifle (and handguns) have taken muzzle loading to a new level. Modern black powder substitutes burn cleaner and produce faster velocities than their predecessors. The breech plugs are replaceable and easily removable by hand for easy cleaning.
Belted, saboted bullets and other projectiles produce unparalleled accuracy. Pre-load kits and powder pellets make loading faster than ever. And what's more important to many sportsmen and hunters--in many states, a muzzle loader can get you into the woods weeks before 'centerfire hunters.'
Add to these advantages the fact that the ATF does not consider a muzzle loader (with a non-serial numbered reciever) a firearm that requires an FFL to purchase and ship. You can order on line and have it shipped to your door. [Some state law may prohibit mail delivery; so, check your state law.]