The point of a circuit breaker is to limit the power that a system receives when current reaches dangerous levels. In other words, a breaker has to sense the max current in the system, and when current rate is above a level, stop it immediately.
So where should a circuit breaker be placed in the circuit to do this most accurately?
The answer is as close to the power supply as possible.
The following discussion is then, WHY?
As power flows through anything, there is a certain amount of it lost to drag along the way. This drag is turned into heat, and results in a voltage drop. Each additional piece in the system can drop the voltage and increase the amperage on any particular wire. For a complete discussion regarding voltage drop on wires, please look here. Bottom line, by putting the circuit breakers as close to the inputs as possible, you get the best "SENSING" from the breakers.