>  Why would triple brood chambers be used as a norm?

Masochism?

As one who ran triple broods for 20+ years, it wasn't due to masochism. It
was actually only following a system that Farrar had designed for two queen
colonies. But if managed properly
It worked well with single queens, as long as the stock had the right
attributes. Which unfortunately current stock does not have, so we abandoned
the system. It has many advantages, 
More feed storage room, which allowed for larger colonies into and out of
winter, when we used to raise our own queens, this system actually carried
drones through the winter and generated them 
Earlier in the spring. Splitting was much easier, dealt with full supers of
brood and bees, no shifting or gathering single frames for increase. The
system generated much larger hives in the spring, and while they consumed
More feed, they had the surplus populations when we wanted them. I actually
miss the triples, and the work it generated and saved.
Tim Townsend
TPLR HONEY FARMS
Stony Plain AB

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