I think the first reason is mostly cost. A 10S4P battery is cheaper than a 13S4P battery because there are 12 fewer cells and at the price point of most hub motor bikes on the market that makes a very big difference.Thinking about the comments on hub motors not being so good for hills where extra torque is needed compared to crank drives, why is it that so few appear to be available at 48V compared to 36V? If 48V is so good why is it not more commonly used? I read a previous thread comparing the TSDZ2 at both voltages and saying 48V was night and day different. Why is say the XF07 or XF08 supplised at 36V if extra voltage would make a big difference to its usability?
Second reason is weight - a 48v BPM is quite a bit heavier than a smaller rear hub motor. Extra weight means that the wheel build needs to be spot on and that also adds to cost.
You will find larger, more powerful 48v motors in the wheels of cargo bikes which cost quite a bit more than your basic 400€ commuter/city bike.