Because the world being created by the Christian God implies the creation story which is pretty different from big bang.
One random source: http://www.catholicherald.co.uk/commentandblogs/2014/10/31/p...
No, the world being created in accord with a literal reading of either of the two mutually conflicting creation stories in Genesis implies something pretty different than the big bang, but literalism as an essential doctrine is, within Christianity, restricted to a particular minority -- though one that is extremely politically active and very well represented in the US -- subset of Protestantism.