Hi everyone; I just discovered this forum, but am a long-time BRP gamer (I bought my first copy of the RQII rules when they first hit store shelves in the late 70's and last played it a couple of weeks ago).
I think one of the sources of D&D's much greater popularity is its modular character: there is a discrete set of character types; they advance in ability on very defined and linear ways; and they complement each other in such a way that each has something different to 'do' in an adventuring party. Setting aside the question of rules, it is very easy to figure out how you are supposed to play a game that is structured like this. It's almost like playing Clue, where you take on one of several pre-defined roles. Ive never seen a variant of BRP that promotes this kind of play. It is easy to make fun of, but is actually really enjoyable and promotes fun group play. So, it is no surprise that people like it.
Another reason is that D&D, in all its incarnations, has had a huge selection of distinct monsters and magic items. So, even if every encounter you play is more or less the same ("What's that weird thing? I don't know, let's kill it! Hey, who gets to keep its zygatoscope of mystical tintintabulation?"), there is great variety in the gooey monsters you encounter and the stuff you get to collect. This too is easy to make fun of, but the proof is in the pudding: lots and lots of people have fun playing games that work this way. BRP could be played this way, but I've never seen a version that really actively promotes it.