It's mainly the cassette motors. The large spline on one side means that if the overall width would be narrower, either the centre would be off-set too far to be able to build a wheel with tight spokes or the motor would be too narrow to produce enough power. You get the same problem if you fit more than 7 gears to a freewheel motor.Well I fitted my wheel as it was the one that came with the kit. From this forum it seems that it is very usual to have to spread the rear stays to get rear hub conversions in. If that’s the case, I wonder why the manufacturers dont make them a little narrower?
Also, may I suggest not using a hammer when bending your frames? ;-)