Possible reasons:
1. The smaller radius on a baitcaster is easier controlled by your stronger hand which in most cases is your right-hand.
Tip: Go to a tackle shop and ask to see a right and left handed baitcasting reel. Crank both as fast as you can. I've found that because I'm right handed, it was easier to use the right hand crank. I'm sure if I practiced with the left hand crank it would also work.
2. It could just be pure tradition. A carry over from England, where everything is ackbasswards. lol
3. Some reels are now maded for right or left handed anglers and some reels are made so you can change the crank to either side.
So many questions so few answers!
Hope this helped