What are the reasons to put gearboxes in the center of the drive train rather than in the front or back? I know that weight distribution is one and the degree of separation for power transfer is lower but that is all I can come up with what am I missing?
In a 6-wheel or 8-wheel center dropped drivetrain, the dropped wheels are theoretically always touching the ground. Direct driving those wheels from the gearbox means that, even in case of a chain or belt failure, you’re still getting power to wheels on the ground.
Other than those two reasons, most teams do it just out of preference. Your gearboxes can go in the front or the back depending on your spacing needs. However, it would be wise to place your battery and (if you have pneumatics) compressor on the opposite side of the robot to balance the weight if you end up going with that. Gearboxes can go anywhere if you design properly for it 
That is what I was forgetting. Thank you.