The tail rotor is more efficient if it is turning in the opposite direction of the main rotor tip vortex. The earlier designs did not take that into effect. Bell flopped the T/R on the 212/UH-1N so they would not have to redesign the gear box to rotate the "correct" way. The Bell aircraft which were designed from the mid-sixties on (206, 222, 407, etc) had gearboxes that turned the proper direction. The AH-1G had an MWO that flopped the T/R with mods starting in 1970.
This is the best explanation for it. The swirl from the main rotor tips rotates in the same direction as the tail rotor thus effectively stalling all the blades at once but only briefly hence the temporary unpredictable power output of the tail rotor and the pilot sees the yaw stability drop where the tip vortices get ingested. It would have been great to have heard what Prouty thought of this but it seems very logical to me. And the rotation direction is just as critical on fenestron as conventional. (The only one that goes the wrong way is Gazelle, every othe fenestron goes bottom forward).
Now if your interest in rotary aerodynamics are piqued, explain why some tail rotors have negative delta 3 hinges.....(increase in pitch with flap)