Because they become disillusioned with their own, as their churches move further and further away from classical traditional Christian norms
Orthodox parishes are growing due to the large influx of people converting from Protestant denominations.
In fact, in recent years, converts from Protestantism are becoming clergy; 79% of new Orthodox priests were previously pastors or parish heads of other denominations.
Why are Protestants converting?
Russian, American and Antiochian Orthodox priests and bishops weigh in on the question.
The long and short of it is that many Protestants come to the Orthodox church because they become disillusioned with their own, as their churches move further and further away from classical traditional Christian norms,
Metropolitan Hilarion Alfeev, an influential Russian bishop even says that the Protestant denominations have experienced such liberalization in the past 10 years “that traditional Christian morality doesn't exist in their Church”
If the 20th century was a century of disillusionment and a worldwide increase of atheism, is it possible that the 21st century will mark a return to traditional Christianity?