They went out of business mostly because Nvidia has been selling cards at , well, between a loss and borderline at cost since the 280gtx, well since the 4870 launched and the volume card, the 260gtx, suddenly had to compete on price with a core that cost much less than half the cost to make, much much much less.
THey weren't the biggest reseller and they, maybe more than others had just been investing at exactly the wrong time. They moved into things like PSU's, which would have cost them a fair bit to get going, and hadn't they branched out in a few other area's, and unfortunately a lot of that investing and expanding basically happened in the months/year up to world economic oblivion. Which meant they, more than most, probably went into the economic crash having recently invested a lot of their cash reserves, being one of the smaller resellers so as sales dropped dramatically and Nvidia(and everyone else) dropped production, they probably weren't getting the best deal going.
Then around that time is also where AMD put a HUGE squeeze on Nvidia pricing top to bottom but across ALL the lucractive midrange cores.
BFG were in trouble, a year before Fermi hit, it had nothing to do with Fermi, nothing to do with Best Buy(other than probably Nvidia seeing an opportunity to replace them so maybe, shall we say, helping push them over the edge?).
I would also guess that, EVGA sell professional cards, did BFG, I honestly don't know which partners are involved in the professional cards sales, I would guess that BFG's professional sales(where margins were still very good even in during the stock market crash and the basically year either side of horrible sales volume) were either non existant or not good, and again this kept other partners going.
Basically, all the partners seem to be having a hard time as sole Nvidia sellers, EVGA has the professional cards, and did have motherboards to fall back on. BFG got more than anything unlucky with timing of their other moves. Theres no certainty they would have stuck around had they moved to AMD, theres more profit but AMD had no reason to give them any stock over their existing partners who were buying everything they could make anyway.
Wasn't it BFG who was rumoured to have bought AMD chips/cards to build their own cards around as a backup/emergency plan, and when it came down to it AMD said no when they asked them to be a proper supplier.
Again at that point, if AMD is selling 25million chips a year, and their partners want 40million chips, they only hurt their loyal customers by taking another company on, and at this point BFG would have been close to bankrupt so you'd also be bringing in a potentially unstable name into the AMD fold where they could quite easily have sold loads of cards, gone bankrupt, and left a lot of AMD card owners screwed, it just wasn't worth it.
