The reason was that the other characters had little or no personality compared to Felix. At first Disney got it right in inventing Mickey Mouse. Mickey was a wild exaggeration, all flailing limbs with no basis in reality. However the move into features though marked a concern for the studio to be taken seriously. However the realistically animated feature film is a contradiction in terms and a limiting of the form.
Animation gives the director total freedom.
There's no waiting for correct lighting or pre-Madonna stars and special effects are the wildest and cheapest possible. The genres only limitations lie in the imagination of its creator. The innovation of Avery is that he pushed animation to its very limits. He gave us eyeballs that literally popped out with lust, a goat that eats the moon, bulldogs being crushed by warships that fall from the sky and canaries that grow to the size of the planet. Disney is often acclaimed for his imagination which, after research, seems inaccurate.