I always thought it was to do with CG, but I could be wrong. I guess there's a difference between piston and turbine as turbines seem to get mounted on pylons (apart from the Comet of course) whereas recips get mounted in line with the wing (apart from the Ford Tri Motor of course). I think a recip is forward of the leading edge so the the engine can be changed/maintained easily and it doesn't interfere with the internal structure of the wing.
So basically, I haven't got a clue.