????
I know of most bikes have gears that go up and down, not front to back.
The only thing I can think of what you are describing is the sitting position of the rider effects where the gear shifter should be placed (sport bikes typically set more in the middle of the bike (seems like it is in line with or "behind" the rider) because the rider's feet are swept back. On other bikes, such as cruisers and touring, the feet are more like a chair sitting position, so the gear shifters are closer to the front of the bike.
If that is not what you mean, well, all I can say is that it is manufacturer discretion and/or tradition.