Because people choose to deny the teachings of the True Church and follow their own flawed judgment.
They believe they hold the absolute truth, despite what the holy fathers have been teaching us since the times of Christ, even though there is much more division within their "churches" than in ours.
This is what protestants are to me: heretics. It's not that I judge them. I actually pray for them and I wish them the best, and I have no say whether they are saved or not. Only God knows that.
I'm not gonna go too deep into arguments about Catholics, because it would take ages to explain why I think they are a perverted Christianity.
The Orthodox Church is the only True Church. She is the bride of Christ.