Why are Built-in/Wall Microwaves Above Ovens when Stacked?
Three reasons:

The microwave oven itself is lighter than the conventional oven, and putting the lighter item at the top makes good construction sense, especially for freestanding oven/range/microwave units.  Such freestanding combo units were a lot more common in the early days of home microwaves, so folks got used to that configuration.
The space over the range/above the wall oven was also generally not used for anything important, and that's still the case.  So homeowners re-making a kitchen will put the microwave over the oven given that the space is &quot;available&quot;.
Items you put into the microwave tend to be lighter than those you put in the stove, so it makes sense for the oven to be lower down.  For example, you might put a whole turkey in a heavy basting pan into the oven, but are unlikely to put anything half that large into the microwave.

Of course, if you're designing your own kitchen, it's possible that none of those reasons apply to you.  Certainly I did not put my microwave over my oven.