Charlie, Turbos are close to the exhaust source, normally, to: 1) reduce exhaust piping lengths, for better response time. 2) to retain as much heat as possible in the exhaust leading to the turbo, more heat equals more pressure equals more boost. #) additional ducting from the turbo results in friction losses to the incoming charge air and, creates a larger plenum effect in the intake tract, which requires more time to bring the boost up (and down.) 4) intercoolers can be used to lower the inlet temperature, it's easier to plumb them in the front of the vehicle, close to the turbo, kind of a logistics problem.