TLTR: Supporting material....What is needed is a really great reason why people who have never played RM should take a chance on the game.
Back in the 80s Arms Law in particular was unlike anything we had seen before.
Having followed this theme in several threads over several years now, and seeing other games coming and going, and upstarts just taking off, it's going to be one of two things. I have to admit, what caught the attention of my group back in college in the late 80's was "Wow! Check out these weapons and tables and these crits!! These are awesome!" And we were hooked.
1) Be something totally new and groundbreaking that everyone says "Whoa, check this out." (A la Weapons and really awesome crit tables. In the already flooded fantasy gaming world, that's going to be nigh impossible, but still possible.
I consider the new Star Wars FRP system groundbreaking. It has some whacky dice conventions, but it's brilliant in the use of the Force and the balance of the Force. While Star Wars is obviously not new, the use of the Force Dice certainly is. A lot of my players have been heavily invested in it and they love the system. At the last gaming session, they pulled in two more players. At our next session, one of the players is going to host Star Wars for four more new players. Just in a matter of 3 weeks, there are 6 existing players, 2 new players, and potentially 4 more new players. The players of Star Wars has just doubled in my household alone.
Devil's Staircase uses a deck of playing cards instead of dice to resolve situations in game and it's brilliant as well. I've told my gaming group about it and they are all checking it out. It's not fantasy world setting like RM, but it's Wild West and that is a rare genre, but the use of playing cards is so different from dice, we're all interested in trying it out.
2) Have really interesting supporting material out there that grabs the attention of players.
This is simple math. There are 1, maybe 2 core books hidden amongst dozens of dozens of other core books for all the different systems. Players look at the cover and maybe flip the book over to read the back, or they maybe see the title pop up in a long list of names on several pages on a computer screen. If we (RM players) are lucky, the viewer will click the name to see what it means. Right off the bat, RM Universal doesn't sound appealing to me so I am not going click it. I don't want another universal system, I want something dedicated and focused. I have GURPS for a universal system. I don't need a second universal system. Given that, I may never get far enough to see the really cool features RMU offers; the armour classes, the weapon choices, the details, the professions, the spell lists, the crits, oh the lovely crits!
What does catch my eye though, is when I scroll through a list online and I see Shadow World: Jaiman, Shadow World: Haalkatain, Shadow World: Quelbourne, Shadow World: World Atlas, Shadow World: Insert Name Here.... I look at those titles and think "What is this Shadow World? There are a string of books for it. Oh, it's a set of add-ons for RMU? What the heck is RMU? Must be pretty popular to have this much supporting material.
Instead of hoping to see "RMU Core book" standing out in a sea of titles, I see a dozen Shadow World titles peppering my screen and I can't help but notice. One of those Shadow World books may really pique my interest. If I really want to use it, I need RMU. Hmmm, maybe I should see what RMU is.
It's just my opinion and thought process, but ask yourself, how many times have you gone through Amazon or RPGNow looking for something new? What catches your eye? Maybe one really cool looking name in a list of other really cool looking names, or seeing lots of support material with a same prefix in the title that supports a game system?