Why, why, Why!

Probably because when you turn right you only have to yield to one lane (one direction) of vehicles and when you turn left you would have to yield to both directions. And if the intersection were such a size, shape, design, or whatever that it would be safe to pull into the intersection if necessary while attempting to turn left after yielding to two different directions then there would probably be a mere stop sign there rather than a traffic light. In other words, it’s for safety reasons.