Why is a wideflange shape more efficient in bending compared

Why is a wide-flange shape more efficient in bending compared to an American Standard? Why didn’t we always make wide-flange shapes instead of American Standard?

Solution

In case of a wide flange, the section modulus of the cross-section is higher.
And looking at the bending equation:

I/y_max = Z
So, as the section modulus (Z) increases, the bending load carrying capacity of the beam increases.

In the American standard the flanges are not parallel to each other, hence it is more efficient in carrying load in both directions (keeping web horizontal as well as vertical).