Here is the answer.Because the word theory has different meaning according to different context.Theory is a contemplative and rational type of abstract or generalizing thinking, or the results of such thinking. Depending on the context, the results might for example include generalized explanations of how nature works, or even how divine or metaphysical matters are thought to work. The word has its roots in ancient Greek, but in modern use it has taken on several different related meanings.
In modern science, the term "theory" refers to scientific theories, a well-confirmed type of explanation of nature, made in a way consistent with scientific method, and fulfilling the criteria required by modern science. Such theories are described in such a way that any scientist in the field is in a position to understand and either provide empirical support ("verify") or empirically contradict ("falsify") it. Scientific theories are the most reliable, rigorous, and comprehensive form of scientific knowledge,in contrast to more common uses of the word "theory" that imply that something is unproven or speculative (which is better defined by the word 'hypothesis').Scientific theories are also distinguished from hypotheses, which are individual empirically testable conjectures, and scientific laws, which are descriptive accounts of how nature will behave under certain conditions.
In short the word theory in science and theory in common usage has two different meanings.This is a common problem in English Language.
Darwins theory of evolution has withstood centuries of scientific scrutiny and answered every question posed by creationists.Its because of this now they are coming up with pseudo sciences like Intelligent Design which always hides behind faith and tries to avoid any kind of scientific scrutiny.
