I suspect a combination of issues (including just the lack of broad support from Chaosium) are involved;
1. First off, its fishing in a different pool than D&D; you can view this in a negative light if you want, but D&D players want a particular sort of high magic, generally over the top fantasy, and BRP and its kin have not, generally, supported that very well; its roots in the relatively realistic RQ have pretty much made that true in any version of it I'm familiar with.
2. While the system as a whole is relatively flexible (as in, easy to add and remove modules for) individual games approaches have often been very paroquial with character generation and subsystems designed to support their particular setting and not easily converted to anything not particularly close.
3. Simply presence of undesireable features for some, or lack of desireable features for others. Once you get into people who want advantage/disadvantage system, bell curve resolution, seperate tracks for lethal and nonlethal damage and other properties that either aren't supported by current versions of BRP or would require actively changing the core structures to support, you exclude more people. I'm not getting into an argument about whether the game should, but those _are_ features some demand, and if you don't have them, you've lost those players.
Individually, most of this wouldn't screen that many people out, but in combination, its not suprising that by the time its done, the market doesn't have a lot of love for BRP. If it gets proper exposure, the new version could help with some of these, but its not going to do much about the issues in #3, and I suspect it can only do so much about #1.