I think many are overlooking the fact that early on in the colonies locally made black powder was inferior to what was generally available in Europe. So, that is probably the reason why American gun barrels grew longer than the European jaegers. Longer barrels were needed to fully ignite the poor quality gunpowder.
Look at the Hudson Valley Fowlers. Talk about a lo-ong barrel. It has been said that these or similar fowlers were probably the first firearms made in America. A long barrel being necessary due to the poor quality gunpowder.
By the time Hawkens came around the quality of our gunpowder had improved.
Of course, now with our modern, highly efficient powders, the longer barrels are not needed. But, oh, what a beautiful, graceful long arm are those longrifles!
Further, the American longrifle, call them Kentucky Rifles, Pennsylvania Rifles, or whatever, were made strictly as a hunting rifle; never meant for the attachment of a bayonet.