@JimmyJ,
Presumedly because they believe that a scienctific understanding of something denies a godly cause of the science. Or, as Dr. King put it so well,
"Science investigates; religion interprets. Science gives man knowledge, which is power; religion gives man wisdom, which is control. Science deals mainly with facts; religion deals mainly with values. The two are not rivals."
In other words, it's not EITHER 'The Big Bang created the universe. Or, "God created the universe." but rather, "God created the universe by way of the Big Bang event."
With evolution, since God has other things to do, rather than making each and every lifeform (bit of a fulltime job) he made life evolve and create itself with evolution.