Duhh… I just thought of another reason the R-1858 might be more popular:
In a recent posting, it was observed that R-1858 replicas were the only ones being produced in stainless steel. Obviously, the replica makers selected a popular model to render in stainless -- and, being available in stainless made the R-1858 more popular.
I'm not a hard-core traditionalist, if given the option, I've selected a stainless model over the blued steel version of the same, or similar gun. This applies to BP pieces and modern revolvers and pistols.