My first BP pistol was an Old Army, and I got it because it offered maximum ballistic performance in a package smaller and lighter than one of the Colt Walker or Dragoon pistols, and it offered adjustable sights to ensure I could place shots on point of aim.
Eventually I moved away from it as I started to develop an appreciation for more historical replicas and their handling eccentricities, and in that regard, I found I was just as happy with a 1860 Army, 1851 Navy, 1858 Remington or one of the horse pistols for plinking, and for target work an adjustable sighted 1858 Remington would do pretty much anything a Old Army would do with just a bit less powder capacity.
I suspect the fact that the Old Army does not really appeal to the replica or reenactment crowd, and the fact that you can get an adjustable sighted 1858 for half the price of even a used Old Army limited the market for Ruger. I suspect a new Old Army would not sell for less than $650, so it would still be twice the cost of an 1858 Remington, and a couple hundred more than a Dragoon replica.