In addition to its location on three easily navigable rivers, why did Pittsburgh become “The Steel City” during the first half of the twentieth century?

it's close proximity with the mines.

Explanation:

Compared to other states, Pittsburgh's is located really close to the sources of mines that produce high quality iron ores. (Steels are made from these ores).

This mean that establishing factories in this city will cut down a lot of distribution cost that steel corporations have to expend. This will increase their revenue.

Because of this, a lot of companies in steel industries started to establishing their base in Pittsburgh, which is why it's eventually known as the  “The Steel City”