Animation is about exaggeration and stylization. Even realism in animation has some degree of stylization to it. Not just the form of the characters, but also their motion (squashing, tweening, etc).
Rotoscoping typically eschews all of that. When I watch something rotoscoped, it often feels like I should just be watching a live action instead. Rotoscoping can be done well, especially if it's light touch, but it's often just a cost-cutting measure that ends up looking terrible.