Reasonably good explanation here.
http://www.snopes.com/history/american/gauge.asp
Has primarily to do with the fact that similar problems tend to be solved in similar ways. Also that people tend to keep using basic solutions even after they’ve lost their rationale for doing so.
As Snopes points out, there have been many railroad gauges used around the world, and even in the USA. Standard gauge has something over half the total.
http://en.wikipedia.org/wiki/Railroad_gauge#Dominant_gauges