Because anything else is offtopic. For some reason people always see fit to bring up their personal religious beliefs when the topic is biology, but it doesn't often get brought up when the discussion is, say, the formation of stars and planets. I don't know why that is, but I know that one tangent is not more appropriate than the other.
If you want to believe in Intelligent Design that is fine, it's a free country, but it isn't a scientific belief and has nothing to do with science.