Sometimes it is a matter of production cost. Some companies simply can't afford the best animators, or perhaps they have decent animators but cannot afford the technology to make the smoothest, most realistic animation possible.
Sometimes it is a matter of artistic choice to make an animation less realistic. Disney has the means and ability to make very realistic animation and CGI possible, but went back to basics to make a hand-drawn animation, The Frog Princess. The film was still very successful because many people enjoy the familiar look of classic animation.
Animation can be a lot like other artwork this way. Just because Van Gogh's paintings didn't look in the least bit real doesn't mean they weren't worth looking at.