The Great Schism was a religious division in 1054 between the Latin Catholics in the West and the Greek Orthodocs in the East. Both sides had different beliefs and liturgies which drew them apart even more so.
Constantinople wanted to be over authority of Rome because it was a much wealthier city, but Rome was a much more apostolic city since Rome was said to be established by St Peter who was given the keys to heaven by God himself. What better city could there be than Rome with this great religious foundation?
With the small exception of Uniate Churches the West and East are still religiously divided to this day.