Because while his (Aquinas') efforts may have been noble and appropriate within the cultural mores of his time, human knowledge/understanding has evolved (i.e. developed, if you prefer) and it is we humans who make up the Church. If the hierarchy decides they need/want to continue to hang their hats on the binary thinking of Natural Law theory, I can almost guarantee one of two things will happen... either a formal schism or the continued, gradual fading of the RCC will occur in all developed nations where freedom is valued and cherished. This is the "real" law of nature and is supported by history, biology, sociology, etc., etc. So in essence, reason and common sense is why you should trust me on this... and not because I said it but, for most of us rather, it comports with our sense of God/nature/truth and the reality of our lived experience.