I love onion rings but most fast food rings are pretty gross. Onion rings take more time and attention to cook than standard fast food menu fried items. Also, rings go limp and soft and squelchy within a short time, so they have to be cooked within minutes to be good. This means there is significantly less profit margin on them. Not saying this is the complete answer but it factors into the corporate decision.
If you love them, make your own, they are way better.
https://www.justataste.com/homemade-beer-battered-onion-rings-recipe/
You're welcome.