McD's, Burger King, Wendy's, Chick-fil-A, Subway, Denny's, Perkins, Arby's, In-N-Out, Culver's, Five Guys, Cheesecake Factory, Sonic, Rally's, TGI Fridays, Jimmy John's, Dairy Queen, Hardee's... the list is a lot longer than I'm willing to spend more than 5 minutes looking for.The Coke one doesn't even make practical sense to me, since virtually every fast food chain except McDonald's is contracted with Pepsi, no matter what part of the country you're in.
The reason it seems like most use Pepsi is simple: Half the rest of the industry is three brands in a trenchcoat called 'Yum Brands'. Pizza Hut, Taco Bell, and KFC are owned by a Pepsi Spin-Off sub-corporation. Same thing with Wingstreet.
Of course I say "Half" like it means anything... Fact is, McD's owns the most fast food restaurants in the world with 41,800+ locations. With half of them in the US!
Anyway. I explained the cultural and historical origin of the tendency. No one has to like it, but that's why it's a thing.