He doesn’t do so because a key issue in that debate is beyond his power to settle--the question of scientific fact. His authority to teach extends to matters of faith and morals, not to questions of science. Some people argue the facts support the theory of evolution; others say they don’t. This is more a scientific question than a theological one.
Incidentally, Catholics holding to a form of evolution compatible with Catholic teaching are also "creationists" (although in a different sense of the word) because they affirm the Creator and his necessity.