Look, the reason that the in-line is popular is because a lot of hunters -- the vast majority of hunters using them -- figured that they could get another season and another tag without actually having to become proficient with a primitive firearm. They are not muzzleloading fans; if anything, they consider BP to be nothing but a PITA and wouldn't ever shoot a BP gun other than to sight it in for muzzleloder season -- see some threads here.
Want evidence? Muzzleloader season, even in some good hunting spots, once had few hunters. The in-line changed that. Sure, there are some guys who have sidelocks also and hunt with in-lines, but the vast majority of in-line sales have more to do with the people who never hunted with a muzzleloader before and have no interest in muzzleloading in general, hunting with historic gun designs, or anything like that.
Now you can think whatever you want about in-lines, in-line hunters, slob hunters, hardcore buckskinners, etc. but that's not the point. The point is that the bulk of sales in the in-line market are driven by the following promise: "Hey Elmer Fudd, you can hunt a whole extra season with a gun that shoots pretty much just like your trusty old centerfire! Just buy one of these!" And nobody uses them for target shooting or plinking, at least not that I've ever seen.
Now what about the ROA? Why would anyone without a real interest in cap-n-ball revolvers buy one? There's no special hunting season for cap-n-ball, at least anywhere I know about. Our F&G laws require single-shots.
Are there BP target competitions? Sure. Are there BP competitions that appeal to people who don't really want to shoot BP? No.
So, as I see it, the ROA market is limited to those with an intrinsic interest in shooting BP revolvers. Within that market, it's limited to those whose purchase is driven purely by the performance of the gun, as opposed to playing with historic gun designs and experiencing a bit of the past. And frankly, that market is pretty limited.
People buy replicas to shoot out of curiosity, and for reenactments, rec room decorating, etc., in addition to just wanting one to shoot like a S&W 686 or something. That's not true of ROA's. Only someone who has a very specific interest in shooting an accurate BP revolver would spring for one -- probably not just an occasional BP plinker or someone who takes his kids or buddies out to shoot BP once a year.
Sales of something like the ROA are not driven by the question, "Will somebody, somewhere, buy this thing?" They're driven by, "Will a bunch of people buy these things on a regular basis?"
The USFA can probably get by on "Will somebody buy this thing?" The answer will be "Yes!" That's one reason it's not cheap.