I suspect it is because there isn't a single obvious 1200km brevet within the North American market.
other than the following (and others)
Big Wild Ride 1200 km (Alaska, USA)
Cascade 1200 (USA)
Colorado High Country 1200k Randonnée (USA)
Gold Rush Randonnée (USA)
Granite Anvil 1200 (Canada)
Last Chance 1200k Randonnée (USA)
Rocky Mountain 1200 (Canada)
Texas Rando Stampede (USA)
Taste of Carolina 1200k (USA)