Because teleology is not empirical. Science relies on empirical observation. Hence teleology cannot be scientific. "Darwinism" is not any sort of science I know of--it is rather a straw man ideology. Evolutionary theory cannot possibly be used to ask the question of design--because design is teleology--and because (again) teleology is non-empirical and hence inadmissible in the field of scientific evidence. Think of science as sort of a trial, if it helps. There's all kinds of evidence you can marshal to make your case, but there are still guiding rules on whether said evidence is admissible or not. The ability to observe, and for multiple parties to repeatedly observe, a phenomenon is the primary guiding rule for evidence admission in the court of science.