Because the observable universe consists of an infinitely large, and intricately interwoven, collection of cause-and-effect series. Every effect is traceable to a cause, which is an effect of a previous cause, which is itself an effect of a previous cause, etc. All these series of effects and causes must, I believe, at least theoretically be traceable back to a First Cause. That First Cause is God.
This is, admittedly, a lousy way of expressing my answer to your question, but it is difficult for me to try to express any concept which involves the infinite.