Why are suspension bridges used for long spans?

The longest bridges in the world are viaducts or trestle bridges.

It is easier to design a high, elevated roadway by

directly supporting it with many towers or piers (as in a viaduct or trestle bridges).

It is more difficult to design an elevated roadway with a large unobstructed span (as in a suspension bridge or arch bridge).

However, the bridges with the longest unobstructed spans in the world are invariably suspension bridges, because ...