A bit late to the party but here's my take FWIW:
When the RM first came out in 1957, it just didn't quite capture the imagination as the Seamster 300 or Speedmaster, so it didn't sell as well. Back then, as now, a watch can say something about what you aspire to be or how you see yourself. Let's face it, for a lot of people an adventurous scuba diver or a race car driver is a more "sexy" image than an engineer or railway worker.
In the early 2000s, the RM could have been marketed as an "antimagnetic" tool watch if it had a soft iron inner case (Remember, this was before the advent of the Si14 balance spring and >15k Gauss magnetic resistance which became standard with Omega). The RM from that era looked the part and was a worthy competitor to the Explorer 1, especially in 36mm case size. Superior magnetic resistance could have been an interesting point of difference compared to the Explorer and the Aqua Terra line and improved its sales.
Sadly, the RM is now a tool watch without a "job" and I don't hold out much hope for a future revival.