Simply because Darwin used well developed (by humans, but well developed nonetheless), well proven, scientific methodology instead of relying on the dogmas of a belief system to base his observations on.
Keep in mind that, in science, there are very few laws any longer. Everything is a theory because, especially after relativity, scientists discovered that our universe doesn't stay consistent as you move into and out of certain states (like the speed of light). However, this does not eliminate the theory as quackery or false. It means that the theory can only be applied in certain circumstances (it's relative to certain states) so it cannot be a "law" as laws are immutable.
That being said, science only has observable evidence as it's foundation (or provable evidence through mathematics). Darwin observed the various species on the Galapagos islands and drew conclusions as to the nature of their adaptations (long beaks, short beaks, etc.). He also observed that various birds with nearly identical characteristics had slight variations in their adaptations. This caused him to draw other conclusions. In short, he used a scientific process to develop early versions of his theory of evolution. It has been scrutinized since, and the only ones "disproving" it have been those who believe religious texts are God's absolute word.
I am a believer in Christ, yet I see no issue with believing there is truth to Darwin's theories. Did God ultimately create all things? Of course He did. However, is the Bible absolute in it's description in the origins of life? Probably not. Even Christ, according to the biblical record, spoke in stories (parables) most of the time so people would understand his message and have examples to work from. He rarely spoke in absolutes, and when he did it was usually only with the apostles.
I believe that Genesis is a parable describing God's creation of all that is, in an understandable way since it is unlikely that we will ever completely understand how He did it. However, even I admit it is only a belief, and may be wrong, like Darwin may be wrong, or literal interpretations of Genesis may be wrong.