The art of the moving characters (not static backgrounds) of old animated movies such as The Sword in the Stone (1963), The Jungle Book (1967), and The Aristocats (1970) were sketchy. The line work of the animation was left rough, instead of crisp and smooth as seen in animated features of earlier and later animated films.

What was the reason for this? Was it technical limitations of animation technology of the time? Was it some artistic trend (if so, is there a name for it)?