Lack of support is a big part of it. And many people do want a game where they can slay rats with +4 swords.
But there is more to it. A couple of points...
Part of the problem isn't what Chaosium didn't do as much as what Chaosium isn't as good at doing. Marketing. A big part of the problem is simply that the Chaosium guys were never as good at getting the word about their game out there and letting people know about it as the TSR or WOTC guys have proven to be.
Also, you can use BRP to create the kind of slay-and-loot games that our +4 sword owners crave...there are a variety of official and houserules that allow the creation of nearly invulnerable PCs. Think about it, Enpeze. In Stormbringer 1-3 you could summon a Demon of Protection with 75 armor points. Thats even better than 75 hit points, because you have to hit with 75 points or better to even hurt the wearer!
And there are other ways. I ran a D&D style game more than once using a pastiche of Magic World and Stormbringer, and it was hack, slay, and loot, just like D&D but with the more playable BRP percentile system. I got my wife into rpgs that way, and she wouldn't even try D&D. Took one look at the mechanics and backed right out, then took to the BRP style 'D&D' like a duck to water.
It is mostly the lack of support and lack (or neglect) of marketing skill. As Charles Green noted, around 1981 if Chaosium had played their cards right WoW could have been expanded and become the first 'universal' rpg instead of GURPS...and replaced AD&D as the base line rpg with a more playable, coherent, and understandable game system.
It also occurs to me that if certain people, like Dave Hargrave, had latched on to RQ or Stormbringer as their original system, the creativity they brought to the hobby could have boosted BRP style systems instead of class and level ones. All these things contributed.