Everyone is going on about how Catholicism isn't all of Christianity, and while they are correct, it doesn't explain anything. Catholicism is still quite important branch of Christianity, and arguably one of the oldest continuous branches of it as well. There's a ton of history behind why the Catholic church is so important in Christianity, even if they aren't the center of it.

The beginning would take us back to the 6th century, when the Roman Emperor, Justinian I, setup a series of 5 "important" bishops, called Pentarchs. The bishop of Rome was one of those. About a century later, 3 of the 5 Pentarchs fell out of Roman hands when the Rashidun Caliphate took over both Syria and Egypt. Then, almost 4 centuries after that, in the mid-1400s, the bishop of Constantinople fell as well. This basically left the bishop of Rome as the center of all Christendom until the next major schism (the Protestant Reformation).

There were also various schisms throughout that history, where different parts of Christianity fell out of communion with others over political squabbles between the Pentarchs, and later national bishops/Kings. There is some on-going effort to bring some of them back into communion with each other, but it still hasn't happened (and likely won't considering some of the demands are a little egregious).