In building construction, it is a highly competitive environment in the cost of materials used for fabrication and construction. Iron was used in the 19th century because of availability and the life time costs, including maintenance cost. As cost for steel came down in installed price, it supplanted iron as a favored construction medium. As higher grades of steel became more price competitive, mild steel itself was replaced by high strength steels and alloys. Other construction materials such as reinforced and pre-stressed concrete and the development of over 10,000 psi strength concrete are providing increasing competition. Even for certain types of facilities, concrete block masonry is proving to be even more competitive. Aluminum can be as much as three times the cost of steel; however, in some instances aluminum can be cost effective. In some chemical exposures, fiber reinforced high density polyethylene is more appropriate.

we used iron for building because it is a more strong then metal .

What the other two answerers said, plus low cost.

I think because it is strong. Also, people use steel.

strong and flexible