I can only speculate.

In the old days it made sense to fly even shortish distances, like 150 miles. As someone said, it was probably a floor to make the program work, i.e. for 150 miles I couldn't be bothered preferring AA over UA, but at 500 maybe I cared.

Nowadays I presume that almost all of such shortish flights are for connection as part of a longer trip, so the minimum no longer makes as much marketing sense.

But, presumably thanks to all of us who complained, AA retained the minimum for elites. ^