You do ask difficult questions! You might as well ask why Kenilworth has six supermarkets, or Chinese as well as Indian restaurants.
The Church in the broadest sense of the word includes Christians everywhere. However, being human, people have always disagreed amongst themselves about exactly what it means to be a Christian or how this should best be expressed. Some points of disagreements are minor, others are more substantial while others are accidents of history.
The result is that different denominations have been set up within the Church of Christ, who take different approaches to the form of organisation and worship within their traditions. As an extreme example, compare the emphasis on mysticism in the Eastern Orthodox Churches with the Calvanist preaching tradition, or the ceremony of a Roman Catholic mass with the intimacy of a Quaker meeting.
The diversity within the churches should be seen as a strength, giving breadth and being a reflection of the infinite number of ways that God may be experienced. To try to homogenise the denominations would be to try to limit God himself.
That is not to say that 'anything goes' and there are some most peculiar sects around calling themselves Christian. However, there are straight forward biblical test to apply both to behaviour: By their fruits you shall know them and doctrine: No one comes to the Father except through the Son.