Everything used to be the mile, for whatever reason historically that distance became the Blue ribbon event (today it's switched to the 100m) of track and field, mostly because of the romanticism of the 4 minute barrier. It made the mile very popular. Why do we run the 1500 m then?
"A metric mile is a distance of 1,500 meters. This distance is used when it is desirable to approximate the statute mile, yet also desirable to use a convenient round number in SI or metric units." - Wikipedia.org
Why approximate?
"In 1976 the International Amateur Athletic Federation decided to only reconise metric distances for world records. Yet an exception was made for the mile." -wikipedia.org
So historically it's always been the mile, the IAAF just wanted to round everything off to metric measurements and since the mile had been such a popular distance/event they wanted to find a metric equivelant (since most of the T&F world lives in the metric system it also helps the casual watcher), thus the explosion of the 1500m and why it is run today.