You don't want fighting dogs smart. You want retarded adrenaline junkies with high pain tolerances that will mindlessly charge to their deaths and keep fighting even after having their eyes gouged out.

A smarter dog wouldn't be as effective at pit fighting.

These traits also mean that it in unlucky cases it only takes one bout of overexcitedness for one of these dogs to go completely nuts and kill someone, and due to their complete disregard for their own pain, it can be very difficult to stop these dogs from attacking once they go into maul mode.