They were pretty common place here in PA when they first came out. But I had the same reaction when I saw one in percussion.
T/C originally made these for PA hunters because their muzzy season was (and still is) flintlock only. And at the time they came out you could only use round balls which is why the PA hunter came with a 1:60 (or 66) twist. A better twist rate than the 1:48 offered in the Hawken and Renegade.