Because Mormon doctrine specifically says no coffee, tea, alcohol, or tobacco. (http://www.mormon.org/mormonorg/eng/basic-beliefs/the-commandments/obey-the-word-of-wisdom). Many Mormons have interpreted the "no coffee, tea" to include cola drinks, but officially, there is nothing prohibiting them.

>Really? How come so many Mormons don't drink coffee? Is it like Catholics who still won't eat meat on Fridays?