Can't add too much to what has been said but I do believe there is a political aspect to MZ exports, that is DDR and the Eastern Bloc were largely self sufficient but they did need hard currency for essential non Bloc spend.
I think in the UK MZ's did fit a gap in the market as cheap and reliable transport and their quality was recognised over time. I suspect the USA was a very different market to the 70's UK and a lot further from the factory!
Andy