One minor reason:
Some might say that the reverse rotating blades will move upward into the main blade downwash making the upward turning blade more efficient. This maybe true but it's minor because guess what the wash does when you fly inverted ...

The biggest reason is the design of the tail rotor (mainly the blade grips). If it is designed to rotate in the reverse direction ... you will have less, little, or no control in any other configuration.

The blade grips are designed with an offset so that when the slider is centered ... the blades will have the necessary NOSE RIGHT pitch to counteract the NOSE LEFT torque generated between the main rotor and fuse. The PROPER offset only exists if the rotor rotates in the right direction, the blades are installed in the right direction, and the blade grip links are in the right orientation to rotation (leading or lagging).

If you simply reverse the direction of the rotor (reverse the belt and tail rotor blades), the new leading edge is in the opposite direction as the proper leading. IOW ... with the control/slider centered, the leading edge of the (properly installed) blade will be angled toward the left side of the heli (providing a NOSE RIGHT command). This also means the trailing edge will be angled to the Right of the heli.

If you simple reverse the blades in the grips and reverse the belt to cause a forward rotating rotor (forward blade moves downward), then what was the trailing edge now becomes the leading edge ... and which way is the leading edge angled now??? To the right (just like the trailing edge was). This will give LOTS of NOSE LEFT command while the control is centered.
See any problem with that???
You'd be lucking to get the heli to stop YAWing left. It would even appear to some that your gyro was reversed (almost identical symptoms).

OK ... what if you reversed the blade grips while you reversed everything else. This will change the off-set amount but your blade leading edge will still be commanding nose left.

As you can see ... the rotor needs to rotate as designed, the grips need to lead or lag as designed, and the blades need to be installed as designed or you will have anything from odd handling problems all the way to uncontrollable handling problems.