After Darwin proposed his ideas about natural selection as a mechanism for evolution, it took time before evolution was generally considered a theory. Why do we now refer to it as a theory?
Fewer scientists agree with Darwin's ideas now than they did at first, so it is called a theory.
Questions still remain concerning its validity, so it must be called a theory.
Enough supporting evidence has accumulated for it to be called a theory.
The meaning of the word 'theory' has changed since Darwin's time, and it fits the new meaning.