It’s their customer service. Polite employees, your order is correct, and if not promptly fixed, and stores clean. Here in Memphis that cannot be said about most other FF businesses.
It’s the product and product quality. For me it’s that their chicken is not all crunchy breading dripping in grease. The batter is lighter and leaves me feeling less sick. The breast is also usually generous in size. Their chicken nuggets are also real chicken not chicken puree poured into a mold like McD.