My guess is because onion rings don't hold up well with time. Fresh, hot from the oil onion rings are wonderful - onion rings left under a heater - not so good.
May also be an allergy thing - I don't know if french fries at WDW are allergin free - but it isn't hard to make a french fry that is free of dairy, eggs and wheat - its harder to make a dairy free, wheat free, egg free onion ring. Since so many people have food intolerances/allergies and because Disney has a good reputation for handling them, they may be looking for menu items that are going to be edible by more people.