First of all, the concept of "unhealthy" of foreign fast food such as McDonald's and KFC is somewhat vague. The so-called unhealthy refers to junk food with high calorie and high fat in the nutritional structure, not to say that it is unsanitary.
Secondly, there is no fast food restaurant in China that is more hygienic and standardized than their environment, and their food can't be cooked at home, so more people will eat it.
Thirdly, it doesn't matter even if they engage in Sudan Red and Instant Chicken. Their chicken is just hormone chicken, not poisoned chicken. It's much safer than other food industries in China, what's more, most of our supermarkets are instant chickens.
Finally, people have gradually become accustomed to foreign fast food, which used to be a taste, but now it is a staple food, the result of globalization.