Because God, to be the absolute creator and the prime mover, He must occupy an absolute space.
Newton had described a universe where, when you push a ball, the ball pushes back. Each action, each motion, is relative to another. This implies that, for God to create the first motion, like rolling the Universe across the room, the Universe would have pushed back and moved God.
Newton didn't believe this, and rejected the argument that his theories disproved God. But there is a reason we remember his laws of physics and forget the theological arguments. His work in physics held up to scrutiny for hundreds of years. He was accurately (enough) describing the motion of the world around him. His religious beliefs were based on the same philosophical musings and wishful thinking as every other theologian.