McD's is a franchise in the US. Franchisees set prices.
[1] https://www.mcdonalds.com/us/en-us/full-menu/5-dollar-meals....
*Prices and participation may vary.
That said, McDonalds corporate isn't running promotions unilaterally. Instead, promotions are proposed by committees elected by franchisees and voted on by franchisees themselves, so participation rates tend to be high.