I don’t think it’s strange, but there’s no reason why they couldn’t.
They used to sell pork spare ribs, and Burger King and Maccy D, plus any number of steakhouses worldwide sell chicken products, as do many fish and chips outlets.
I suppose it’s purely a marketing decision.
They want to be known solely for their fried chicken and clearly think that offering pork, beef, lamb or fish products would not make them a profit. They’d also need to re-equip all of their kitchens to handle and prepare red meat products separately.
They’re also not Kentucky Corn on the Cob, Kentucky Coleslaw, nor are they Kentucky Vegetarian Quornburger (a.k.a. “The Imposter”), but they sell those products.