Read your history. Council of Nicea?
One Christian church until about 1,000 A.D.
Roman Catholics then separate.
c. 1500 A.D. the Protestants separate.
The Roman Catholics separated from the Orthodox, then the Protestants separated from the Roman Catholics. The Orthodox kept going in a straight line.
It's a simplification, but there you have it. The Orthodox refuse to change. It's even a joke: "How many Orthodox Christians does it take to change a light bulb?"
Change?! (Meaning that it's strongly resisted.)
Btw, I'm saying this as a former Protestant. I still think as a Protestant. Orthodox thinking is at once deep, odd, sideways, and very often deeply insightful and satisfying. I'm still getting used to it.