Creationism, by definition, is the belief that all things were created in their present form
. Species are immutable, can not change.
As such the concept of evolution runs entirely counter to this.
Now, for people who are not religious, or whose religion admits to the possibility that creation legends are illustrative metaphors rather than cast iron facts, this isn't a problem. Evolution can be successfully incorporated into the overall understanding of the universe. So, we evolved? Well clearly that is how God planned it.
For others, lacking such flexibility, evolution is a total denial of one of the most basic parts of the belief system. And if one part isn't true, how can you believe the rest?