Just my opinion, but you tend to push the stuff into the saw with the right hand, and therefore having the fence on the right enables the left to be used to guide / keep it against the fence.
The mitre slot of the left keeps you away from the blade, you have a stronger grip with your dominant hand and you tend to use it naturally to hold the stuff nearest the blade. If the slot was in the other side you would likely have a hand either side of the blade......my Startrite saw has slots on both sides.....when I used the RH slot one day I got that 'what the heck are you doing being so dumb' feeling and soon stopped!!