Why was it difficult for Dalton to prove his theory?

His theory like the Aristotelean epicycles theory assumed motions were based on circles. Thus even though his theory was simpler than Aristotle's his predictions did not agree as well with measurement. It was not until Kepler changed the circles to ellipses that Copernicus's theory gave predictions that agreed better with measurement than Aristotle's theory did that it was finally easy to accept the Copernican sun centered system.