Shall we talk about Fabula Ultima? Gorgeous book. Tiny publisher. Currently an Adamantine seller.
I didn't want to get involved in this discussion about apples and oranges, but I have to second that. Fabula Ultima is a great book, with very nice art and a layout I find quite conducive to reading.
It also happens to be a (personal opinion again, obviously) quite interesting RPG, which I believe is a great part of the appeal. But it's one thing to be great, and another thing to be accessible and make the greatness be obvious. FU, in my opinion, does that : the book art and layout makes it easy to grasp the core of what makes it interesting.
Now compare it with RMU and see where the differences are. It does not explain the full difference in popularity, but it probably does explain part of it.
I must admit that the last RM book I could read without wondering why it took so long to get to the freaking point was probably the original ChL&CL. RM2 suffered from a critical lack of editorial and quality control, and from RMSS onwards, the core of the rules and the design intention were buried in meandering prose. Personal opinion here, but the first commandment should be "get to the point"; the second should be "provide an example"; and the third should be "provide *technical* design information" (instead of an essay on the relative values of historical and literary source material, which can easily be omitted or moved to the appendices).
Layout and artwork is the icing on the cake.