I think that the IBA's more focused on longer distances ridden at a slower average speed - shorter ones have a greater chance of turning into high-speed sportbike rides rather than endurance rides, which is to say both unsafe and not entirely cool.
The BBG (Bun Burner Gold - 1500 in 24) and permutations thereof is about the hardest IBA ride for that reason - it's not about just twisting the throttle, but managing time, fuel, distance and planning, available light, food, fatigue, etc. The BBG just has the lowest margin for error as such.
They also like destination or themed rides. The 48/10 is the biggest, but there's also the Great Lakes, CC50, Trans-Canada, in-state/province Saddlesores, Heaven to Hell, Top-Down/Bottom-Up, Border-to-Border, etc.