Because the universe had a beginning.
Something which has a beginning does not come from nothing. There has to be something outside the universe which caused the beginning, and for anything to exist at all, there has to be something at the source of everything which always exists, and that is God.
If you think, as I suspect you do, that your God is a handy way of avoiding the infinite regress problem then how do you
know that the Christian God didn't itself have prior causes such as a series of even greater preceding and creator gods prior to your personally preferred choice of god, where an earlier god than the Christian version is the 'real' first mover?
Even if you go down the 'God is eternal' route then you'll need to explain why other things (like universes) can't also be 'eternal', and do so without your usual descent into a pot pourri of fallacies.