There is also the problem that Inner Beast is a GCD skill, while TBN is off-GCD. It's more difficult to activate IB in time. When you just hit an attack button when the tank buster cast starts it's nearly impossible to activate it in time, except it's a really slow cast.
On paper IB is maybe better, but in the real fight TBN is more reliable, especially for the standard player who plays "on sight".
