It's all about weight & balance. You need to distribute the weight (i.e. the Self Loading Freight & bags!) evenly through the aircraft.
As stated, most people given a choice will choose to be near the front, so those who don't choose tend to end up towards the back to counterbalance those who choose to sit at the front.
Obviously bags (J2 load the rear holds first) have an impact & enable a larger % of passengers who want to sit towards the front to be accommodated.
The 733 isn't a bad piece of kit trim wise - the QCs are naturally a little more nose heavy - well 1 ton plus of cargo door would have an impact!!