If there is only one God, then why is there so many "different" religions an beliefs about the "same" God?
From a Christian perspective, man is naturally religious and therefore man desires to know God. Over the course of many centuries, man has therefore come up with different religions to describe what God is like and what God requires.
Christianity believes that man through reason can know that God exists, but needs special revelation from God in order to truly know what God wants. The central claim of Christianity is that God himself has taken human form in Jesus Christ and revealed to us what he is like.
In other religions, men tell us what God is like and what he wants (hence the differences in religion based on men's opinions). In Christianity, God becomes a man to tell us what he is like and what he wants.
You might just as well ask, As my country has only one government, why are there so many political parties? Why are there so many different theories in physics or chemistry? Why do people have different ideas about economics?
If people had perfect knowledge, then by definition we would all know the truth about God and we would all believe the same thing.
But we don't have perfect knowledge. People disagree about lots of things. Different people formulate different theories to explain the observed facts. Religion is no different than any other field of study. In the end, on any given question some people will turn out to be right and others wrong. Or maybe it will turn out that we're all wrong and the truth is something that no one thought of.