What is with chicken that it is used most of the time as the "main" product of most fast foods, specially in the US? Is chicken easier to prepare? to cook? What is with chicken?
In a global view, chicken is the meat product least likely to be objectionable to various sub-groups (other than vegetarian/vegan sub-groups.) Beef and pork are taboo foods for various religions and this will presumably alter the menu in locations where those religions predominate. I'm presently not aware of any group which eats meat that deems chicken unsuitable.
KFC, previously known as "Kentucky Fried Chicken" will, of course, be chicken-centric. They have other sub-brands (i.e. Taco Bell) which sell other meats.
Chicken is hardly the predominant meat product in USA McDonalds. They sell mostly ground beef patties.
Your premise is false: while KFC mostly sells chicken (because they are a chicken restaurant), other fast food providers, including McDonalds, do not predominantly sell chicken (see this article listing the top 10 most popular items at McDonalds in the US, only one of which contains chicken).
For instance, this article lists the top 20 fast food chains in the US, of which only numbers 8 (Chick-fil-A) and 11 (KFC) serve chicken as their 'main' products.