Question

Why is the freezer located on the top of the refrigerator?

Answer 1

Initially or even now in some cases the freezer was actually inside the refrigerator cabinet. It is the coldest place in the fridge where the cooling coils are located, and when cold air leaks out of it, that cold air falls down into the outer cabinet of the refrigerator.

Warm air from the refrigerator portion rises up to the freezer section and gets cooled in turn. This is also why the vegetable "crispers" are located in the bottom of the fridge, I believe--the coldest air from the freezer falls there first, keeping the fruit and veggies freshest.