That's an excellent shittyaskscience answer.

However, it's more about sunlight. The sun's rays hit more directly near the equator (generally speaking) than toward the poles. All people need sunlight, and we all need similar amounts of sunlight. Thus, it only makes sense that taller people live further from the equator so that they don't get too much sunlight, while shorter peoples had to migrated toward the equator and continued to evolve there as shorter people because they had less need for the additional surface area to catch extra sunlight.

I think this can be pretty readily seen in the abysmal levels of Vitamin D in people in developed countries in recent years. We are a mobile people, and we travel all over the globe. We're no longer confined to the varying amounts of sunlight in our geographical homelands, yet haven't had time to evolve to need less Vitamin D, which is produced by the skin in response to sunlight.