Which is why the lines are non existent at the "Song of the South"-based Splash Mountain.
Dumbo is indeed a classic film that can appeal to everyone, but the ride had and has almost nothing to do with the actual story of the movie itself. That's part of what's great about it - you don't have to be familiar with the film to enjoy the novelty of riding in an adorable flying elephant around Fantasyland.
It amazes me how little Disney seems to understand about how the intellectual properties in the park interact with the guest experience. In the past two years they have both closed an only mildly popular Snow White ride and are about to open one that will do gangbusters. This is to say, clearly the property does not matter nearly as much as what you do with it. That's the secret to a successful theme park experience. Just make something great. Dumbo the attraction, simple though it was, had enough elements that added up to a great experience - the view, the flying element, the cutesy ride vehicle, and yes, to some, the wonderful film tie in - so people waited in line.