Welcome Stockman. Maple is very prevalent here in America, curly maple is tree to tree ......... where you find it. Curly maple is not a species, but is an anomaly that occurs in maple trees occasionally when the grain, for some reason, grows in an irregular, but pretty pattern. I cannot say definitively why so many Kentucky style rifle were built with maple, over walnut, because American black walnut is very common and widespread as well. But, I'll hazard a guess. Having worked with both walnut and curly maple (there are several curly maple longrifle blanks resting in my shop right now) I believe part of it may be that, IMO, maple is a bit lighter than walnut, but stable, and with a longrifle that would be important. Curly maple with very tight curl is not easy to inlet at all, but it can be done with patience, else I would not have been able to pull it off. The old time gunmakers had a lot more patience than most of us have today.