Ok, that thread didn't answer the question.
First off, fender mounted mirrors on factory cars for the most part are on Japanese cars, not European or American cars (though there were some vintage exceptions to that rule, as the '50s cars mentioned in that thread indicate, and a couple old british sports cars like Morgans).
On the Japanese cars, the mirrors were fender mounted due to governmental regulations stating that the rearview mirrors both had to be viewable through the front windshield INSIDE the swept area of the windshield wipers. This was for safety reasons. This regulation was in place from the '50s through 1982 on passenger cars, and much more recent on commercial vehicles. In 1982, Japanese cars started moving to A pillar mirrors in search of better aerodynamics for fuel economy reasons. It took another year for all the models to adopt the newer style.
Check out some of the changes here: http://db.tokyo-motorshow.com/cgi-bi...y/form_en.html