Enpeze- There are several reasons that BRP failed to gain popularity.
Read Shannon Appelcline's history of Chaosium on RPGnet.
They did some things that were not so great including failing to convert Runequest II into a universal system soon enough and supporting it.
Also TSR/WotC has been the big boy on the block for quite some time. It has controlled 40-50 percent of the gaming market for years. Those products are very likely to be what a person new to gaming is exposed to and, as Trifletraxor points out, that oftens sets peoples' expectations of games. BRP has no levels-What? It has no character classes-What!? All skills and magic are available to any character-WHAT! Heresy! It doesn't matter that the rules make better sense and model things better- they are different from what the person knows.
D&D also serves as a gamer right of passage. Nearly every gamer has played some D&D in their time. Often there are fond memories of particular modules which serve as a basis for bonding and comradeship between gamers or at least as material for boasting about how fast the party took down the evil XXX ...or how fast the party was eliminated by same!
BRP currently lacks such material.
Currently the universal system niche is being filled by GURPS and HERO and every other game that a GM prefers to use for a setting because it games well and not neccessarily because it models things well.
Other games that have done well seem to do so because they are structured around a compelling story/background. Vampire for instance, Exalted, Ars Majica. Glorantha derives a lot of its market share from people that are hooked on Stafford's world. They will jump systems in order to continue to get it. See the people buying from Mongoose? It willbe interesting to compare sales of MRQ rules to sales of MRQ Gloranthan supplements.
GURPS and HERO both have worked hard to bring material out that would allow the buyer to model particular genres. GURPS went further and produced books that allowed buyers to participate in adventures set in their favorite literary or media work. That seems to be just as important as producing a good set of rules.
Chaosium was doing the same thing with Elfquest, Stormbringer, CoC, and Ringworld. Then things blew up and they lost Ringworld, Glorantha, Pendragon, and stopped supporting ElfQuest and Nephilhim. They were left with CoC and Stormbringer and too small a staff to work out other things. It would be a big gamble to produce things that did not support the core products so Chaosium became Cthulhu Central with a dash of Elric thrown in.
BRP currently lacks such material.
Advertising- Roleplaying has gone mainstream and the big companys are advertising and can get their games in chain stores. Chaosium probably cannot afford to do so.
I think that a better question is what can we do to help popularize BRP?
I have an 6 point plan:
1) Buy it.
2) Play it.
3) Run it at conventions.
4) Talk about it on forums.
5) If there is a weakness in the rules produce fan material to patch it. You might be the next "Perrin Conventions" dude. Or Dudette.
6) Produce material useable with BRP.
Joseph Paul