Well, I travel all over the world, and I occasionally eat at "the American Embassy"
1. 99% of them, even in the dirtiest countries, have a serviceable restroom
2. Despite the jokes, most Mickey D's are sanitary in food prep. In places like India or Indonesia, this can be important
3. It is familiar. After a long road trip through several countries, quick and familiar can sound pretty good. I am adventurous, but a week of Indian food, Chinese, or any other cuisine can lead you to just want a burger, no matter how much you like local foods
4. As an expat, sometimes you just crave American style junk food
5. Mc Donald's represents the USA. Though far more culturally aware than most corporations, and different menus around the world, the standard Big Mac etc are everywhere
6. Believe it or not, mc do is almost obsessive about quality
7. It is, overall, cheap. In expensive countries like Japan or Norway it can be the cheapest meal around