TO understand BB30 you need to understand the square taper -> isis/octalink -> Outboard bearing/Hollowgram transition.

Square taper- small spindle, Larger bearing balls, normal Q factor

Octalink- large spindle, but needs smaller bearing balls to fit inside the frame's BB shell, Normal Q factor.

Outboard bearing- Large spindle, larger bearing balls, but wider Q factor.

BB is the next evolution in this thinking, one that allows for a narrow Q factor yet retains the larger bearing balls and larger spindle of the Evolutions since ISIS.

BB30 is also the first design to take on a new BB shell (other than some oddball solutions, like Gary Fisher's sealed cartridge bearing BBs that popped up in the mid 80s. Right?