Everything I see has the reason as being too costly to retool for right hand drive, and the different safety and emissions regulations, which often vary from country to country. A range of engineering specifications from how headlights are configured to the height of a hood need to be met before a vehicle can enter a market.
Not saying that you are wrong, just that I can't verify it.
SOURCE of above, but I didn't do a straight copy & paste of all of what I wrote above.
I originally got the info from a book about the History of the Mustang, I read a few years ago...................
only UK is right hand drive, rest of europe is same as us