It is always difficult to know exactly why a religion comes out the way it does. Adherents of the religion might say that it came out that way because that was the will of God. We can only speculate.
The basic structure of the Catholic Church is and was hierarchical. Early on, the structure was not as elaborate as it is now, but it involved bishops who were local leaders over elders and deacons and presbyters of the individual churches. Later, there came to be general agreement that the bishop of Rome was the head of the Church.
This arrangement can be said to have arisen because of the difficulty of communication in early times but also because of the need for the Church to have a single message. Because of the difficulties of travel and communication, it was necessary to have local authorities. This may have also been necessary because the Church existed in many different places with different cultures.
At the same time, there was a need for hierarchy. Christianity was a minority religion, often persecuted. Because it was a struggling faith, it was important to try to get as much unity among Christians was was possible. This can be seen as a cause of the rise of the hierarchy.
Something else that is very important to mention here is Matthew 16:18. From the New International Version of the Bible, this verse reads as follows:
And I tell you that you are Peter, and on this rock I will build my church, and the gates of Hades will not overcome it. Matthew 16:18
This is important to the "creation of the basic structure" of the Roman Catholic Church because this is when Jesus started the Church. More specifically, it is where Jesus chose the very first Pope. All Popes in the Roman Catholic Church (all the way from Peter to Benedict XVI) have come about through "apostolic succession" ... which led directly from this very moment.
Any discussion about the creation of Christ's Church cannot be complete without this simple bit of information.