Because to some people it isn't just a scientific theory. Its the basis of a whole worldview, a whole philosophy, that focuses on the natural world to the exclusion of God and anything beyond the material. Simply put, if you believe in Creation, then you must believer in a Creator, and in the possibility that He might just have an opinion on how we should act.
Honestly, I think this is really what has caused the backlash inside Christianity (And to a lesser extent, fundamentalist Islam) against secular society in general and evolutionary theory and scientists in particular. It isn't so much the theory itself - ask the average Creationist to explain carbon dating, or how oil formed, or how fossils work, and he'll probably only be able to give you a rudimentary answer. It isn't the science itself people have a problem with, it is the implications and the philosophy that others have brought with it. Many evolutionists have an almost religious fervor in their defense of the theory of evolution, and it is kind of disturbing.
I think it's a shame that what should solely be a scientific issue has instead become a religious and philosophical one. How exactly we came to be, biologically, isn't really important to most people, or to Christianity - shouldn't you still love your neighbor, even if he is descended from an ape like you? - but the materialistic God-denying philosophy that unfortunately so often comes with it is definitely a threat, and is what causes so much trouble. I don't think science is the problem - as I've said, most people don't understand the science involved, and don't care too much about it - it's what has been unfortunately and unnecessarily added to it.