Why might a scientific theory change?

Evidence is gathered or collected that refutes the theory. The theory must be discarded, or modified to fit the evidence.

Examples of discarded theories: Phlogiston, Ether, The Steady State Theory.

Newton's theory of gravity has not been discarded, simply because it remains useful, even though it has been replaced by Einstein's Theory of General Relativity. GR could be considered an extension of Newtonian physics. With Darwin's Theory of Evolution, the original theory has not changed so much as it has been extended by new findings. We understand the structure of DNA much better now, and the role of genes. DNA was unknown during Darwin's time. Molecular Biology, genetics, and cladistics simply build upon the underlying theory of evolution.