Why couldn't evolution co-exist with the idea/belief of creationism?

Evolution does exist. What's open for debate is how much evolution plays a role in our (indeed all life's) existance.

It's proven that giraffes once had shorter necks. Their bodies adapted to the environment just as humans have adapted to their environments. Different races amongst our own species is proof of evolving to adapt to the environment. African natives tend to have larger nostrils to facilitate sand filtration in desert-like climates. Their skin is also, clearly, darker due to millenia of walking in the sun. Just as Innuit have a propensity for chubby cheeks; they live in polar regions where excess of fat helps maintain warmth. There are environmental reasons why Skandinavians tend to have fair hair, eyes, and skin, as well. The list goes on. Mankind didn't just become like this, they evolved to match their environment.

Small scale evolution is irrifutible. It's proven that bacteria evolves to match environmental changes. There is type of month in Hawaii which subsits solely off pineapple trees yet, miraculously, the insect was introduced to the islands only 50 some years ago, before such time they had never encountered pineapple trees! Their diet and digestive system evolved in a very short time.

Large scale evolution, I agree, is open to debate (though certainly one possibility among many -- nothing is for certain). Whether mankind simply evolved from single-celled organisms, into monkeys and, thus, into homo sapiens is certainly unproven. Though there is proof of subspecies of pre-homo sapien humans (neanderthal, et al).

I don't see why evolution and creationism necessarily need oppose one another. Belief in (a) God doesn't require denying evolution.

Just my two cents.