You might as well ask:
If first believers were called Christians, why are there Catholics, Baptists, Methodists, Calvinists, Anglicans, Orthodox, Evangelicals, etc.
Answer: They are Christians. All of them.
I disagree. Church membership and Christian (Christ follower) are not equivalent. Yes, there are Christians in all of the above but not all are Christians.
“They asked him, ‘What must we do to do the works God requires?’ Jesus answered, ‘The work of God is this: to believe in the one he has sent’” (John 6:28–29).
The answer is, there was no Pope. Christians don’t have a Pope, Catholics do.
Baptists, Methodists, etc are flavors of Protestant Christians, which were created out of the lapse of good doctrine in the Catholic church.