Why? Room service in my experience is more utilized in business class hotels, where people are on work trips, many times solo, and are getting meals to the room either to continue working, or they don't feel like going out to a restaurant by themselves.
How many people are at WDW that are 1) staying in their rooms during regular meal times? 2) would rather sit in their room and eat as opposed to eating in parks/restaurants; 3) Want to have a family of X eating around their beds as most hotels aren't set up with area's for a family of 4+ to eat in them.
People who eat in restaurants generally order more. They have another round of drinks, they decide they do want the desert, ect. Why would WDW want to staff the infrastructure for room service without a significant value add. Especially when you consider that with independent hotels, there is at least a value of having people eating in their rooms, as opposed to leaving the hotel and going to an independent restaurant. IN WDW unless your driving off property, your alternative dining locations are going to be in the bubble and WDW isn't loosing out on any revenue.