Why has the world not yet agreed upon using one set of units (i.e. meters for distances and kilograms for weight) everywhere?
In large part the world has decided to support a single standard of measurement. The metric system.
Only three countries have not adopted the metric system.
-U.S. 
-Liberia 
-Myanmar

In the case of the U.S. the existing industrial base is too well established to switch standards.
It would be prohibitively expensive to rebuild the industrial infrastructure and retrain workers on a national scale.