Good question! Most of the tundras of the world ARE in the artic circle, but as you noted, they do occur in the Andes Mountains and the Himalayas, which are both very high mountain ranges. The reason for this is because as you go up in altitude the temperature gets colder. Air pressure also decreases as altitude increases. (So not only do you get colder the farther you hike up the mountain, the air gets thinner too!) These two things are connected. As the air gets thinner, the molecules spread out, which cools them. Now we have a bunch of REALLY cold air on top of the mountain, now to deal with the little precipitation part.
When you have a mountain, sometimes you have what's called a "rain shadow effect." Basically, you have a wet area that a prevailing wind travels over and picks up moisture from. The prevailing wind, now loaded with moisture, blows across the land till it comes to the mountain, then blows upwards. As the altitude increases, the air gets colder. Cold air can hold much less moisture than warm air. So, the air cools and cools, until finally it can not hold the moisture any longer so it rains on one side of the mountain. Then the now- dry air continues it's way up and over the mountain. One side of the mountain gets rain, the other side is very dry. If the mountain range is very tall, as in the case of the Andes and Himalayas, it rains before the air gets to the top of the mountain. There is a figure on wikipedia I have included a link for that expains it better with a picture. Hope this helps!
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