Long story, and it boils down to engineering preference at the time of design, followed by inertia afterwards. Some of the current designs were made in the late 50's/early 60's and have stayed the same. About that time someone did research that showed that top turning aft was the optimum direction, but those with the tail rotor top going forward haven'g changed as they'd have to re-qualify the design, and re-do the flight test - all very expensive.