I'm not sure of the time line for RUSA's creation, at one time the full history of randonneuring in America was on its web site but now it's only since the 'breakaway.
I'll do some digging, but for now I'll say that the present RUSA organisation broke away from the original because of egos - what's new? When that occurred I don't know, but enough to assume that BMB preceded that event.
RUSA is such a crap organisation today that another 'breakaway' is due. The amount of dissatisfaction with the RUSA board among the membership is another cause for its decline, as is the peculiar Regional Brevet Administration system whereby one person only in a geographical region can decide the brevet calendar and and only that person can design brevets.
It would be as if in Norfolk for example, one person living in say Dunstan organised all Norfolk brevets and it was that person who decided where and when the brevets would start and on which routes they would be run for the entire county. You can imagine the Little Hitler complex that person could - and often does - develop and the lack of interest someone living in Cromer would have to participate.