It's not just the pope, all of the Catholic priests are supposed to be celibate. I read this book by a professor who studied early Irish history and his books are mysteries set in the period that he studies so his historical statements in his books are based on his research.
He states that celibacy was started by the Catholic church because of money. If a priest was married and had children, when he died the church would have to support his wife with a pension, and his children while they were under age. Similarly, any property that he had would be inherited by his wife or children. If he was single then the property would go to the church (of course they pressured them to write their wills that way). A lot of priests came from wealthy and titled families; the oldest son would inherit everything so the younger sons had to make their own living and the priesthood was reasonable choice for them.