I think there are a few reasons:
1. Many people use the "truth" of evolution to doubt the validity of the scriptures.
They say "I know science tells the truth, so if the bible disagrees, I know it must be wrong." Xians want to remove this intellectual stumbling block to faith. Not that you have to believe in creationism to believe in Christ, but evolution should not be seen as a valid objection to the claims of scripture.
2. Interpreting the OT as compatible with evolution is considered by many to be poor scholarship.
While some people think that evolution and Genesis can be harmonized, I think they can ONLY do so by interpreting the OT as metaphor or archetype. While this may be acceptable for homiletics (teaching), it is not really sound hermeneutics (interpretation). That is because one of the main rules of hermeneutics is that your primary interpretation must be done with respect to the literary type. For example, if the passage is obviously written as poetry, it should be interpreted primarily as poetry, not historical narrative. The genesis accounts appear to be historical narrative, and so, should be interpreted as such, i.e. more literally, not just figuratively or metaphorically. However, not simply literally, as in "like a science textbook."
To be even more detailed, they are not just historical narrative, but ancient Hebrew historical narrative, so there are even more rules to apply – for instance, the repetition of the creation days in Genesis 1 and 2 may be understood as a common Hebrew device to emphasize different features of the creation – so the "contradictory" sequences might not be contradictory because only one may be chronological. So an overly literal interpretation might expect both passages to be chronological, and therefore contradictory. I could go on, but I am getting out of my depth here – the point is, conservative xian scholars believe that these passages should be seen as historical narrative, not just metaphor. They don’t think that the "day-age" or other metaphorical interpretations are correct if traditional rules for interpreting these texts are used.
3. Evolution devalues humanity by making us merely animals.
Xians argue that viewing man as an animal removes his responsibility to treat human life with more respect than the other animals.
4. Evolution’s resulting philosophies disagree with Xian theology.
For example, we may infer from evolution that death and sickness are just part of the divine plan of evolution, that is, what God intended, rather than the results of man’s disobedience and separation from God. I’m sure there are many other such ideas that flow from evolution that theologians would take issue with as "anti-biblical"
5. Evolution is seen as supporting an atheistic world view.
While there are many xians who are evolutionists (theistic, I suspect), many xians object to a theory which so easily lends credence to, or provides a theory of origins that can form a foundation for a "godless" world view.
6. Evolutionists hold to many other anti-biblical views, such as doubting the Noahic flood.
Unfortunately, evolution is often bundled with many other scientific doubts surrounding events in the bible, such as doubt in a world-wide flood, the lifespan of the patriarchs (800 + years each) or doubting such "miracles" as the fall of the walls of Jericho, or the destruction of Sodom and Gomorrah, etc. So evolution is often seen as the big star in the anti-biblical arsenal – as this idea falls, the others may also topple with less effort – but as long as evolution stands, it holds up the other "anti-biblical" ideas. Admittedly, I’m not sure about this one, I just thought it up while writing – but I think it has some truth to it.