When my '73 Nishiki Pro was assembled new, the mechanic was an old, exbike racer from Italy. He set me up with R hand -front brake and I have set up every subsequent bike this way. It seems the most logical and I am a right handed person.

As has already been posted, when the side pulls are routed this way you get the shortest, most direct cable routing.

Also if you are running DT friction shfter (as in Campy NR) and are shfting the RD you can still slow in a group w/o going over the bars as you can engage the rear brake with the left hand while shifting with your R.