There is the theory that Rohr was an aerospace company and didn't know about standards; I find this hard to believe. Some say there was a study about car dynamics. But the article I read said that no one actually remembers. The design docs are in a transportation library in Monterey and they apparently don't say. Here's a collection of theories, all plausible with no confirming evidence:

http://myprogressiverailroading.com/rail_forum/maintenance_o...

BTW, Pittsburg uses 5'6" as well.

DC Metro is even more ridiculous. It's only 1/4" different from standard gauge.