I'm not saying this is why MS chose to go with the power brick but keep the following in mind:
By moving the power supply to outside the case, there is less heat generation within the device to deal with which can lead to quieter operation. Also, there is more open space within the console itself which further helps with heat. It is also modular which means service quicker turn around.
Again, I don't know exactly why MS chose to go that route.
MS might be a money grabbing whore of a company, but they won't do stupid things intentionally. Given this is their third console, there has to be a reason.
One can presume it was because either:
A. It helps with heat.
B. It was cheaper to manufacture and saves MS money and thus increases profit.
C. It saves money down the road in service costs by being modular.
D. A bit of both everything.
I bet the answer is D. In truth we'll never really know, but whatever the reason, it probably made sense in whatever context the decision was made in.