There are many reasons.
When I was a kid, I shot black powder, because I thought it was neat, and because I could afford it. I recently ran the numbers, and while I still shoot some Black because it's fun, I can probably shoot factory .38's for the same money. If I bought a new $500 Ruger single action revolver these days, it'd be a Blackhawk.
When I was a kid, most replicas were junk. The ROA was so much better than just about anything on the market, people were willing to pay for it. Replicas have come a long way in terms of accuracy, quality and durability, they provide a genuine "old-fashioned" shooting experience, they're still not too expensive, and they look better than ever.
When I was a kid, SASS was something your momma slapped you for. Shooters with historical interests now have a huge "scene" and a lot of competition, friendly and serious, in the world of CAS. Ruger now sells a lot of New Vaqueros, not ROA's, to the demographic in question.
In sum, Ruger had a great product and could get a high price for it, 36 years ago, when I was a kid. Ruger's product stayed the same, while everyone else's improved. There wasn't anything wrong with the ROA, but whoever wanted one probably already had one. There has been a shift in demand for a modern redux of a 19th-century revolver from black powder to cartridge, and Ruger has been there to cash in on that demand. The production line is more profitable making New Vaqueros.
Now I'm not saying what they should do, but I figure those are some good reasons...
Maybe I should buy some Ruger stock, while it's in the toilet. Seems like the management wants to run a business, again.