They have no idea what Orthodoxy is. You'll find that Protestants are still a bit hung up on their Mother Church. It stems from the 500 year tradition of attacking the Romans began by Henry VIII.
Most people do not know what the orthodox church is, or that it is one of the oldest churches. Its older then the Roman Catholic church. The Catholic church broke off of the Orthodox in the 1000's AD
Since I was raised protestant and have yet to convert away from it although I plan to I can say from personal experience that they don't because most don't know it even exists. The only reason I do Is because I grew up watching the history channel for much of Holy Week every year
The beef between Protestants and Catholics is an old one, but the Orthodox and Catholic churches split long before, officially in the great schism, 1054, in which they each excommunicated the other. The thing about Protestantism is that it comes from Catholicism, but not Orthodoxy. The Orthodox have had their share of heretics, especially in Russia, but none of them were very orthodox. The thing about the reformers is that for the most part, they were right. The medieval Church had strayed far from its catholic roots. People like Luther just wanted to get back to the scriptures and the Church fathers. He didn't say anything that wasn't said by the fathers, at least at first.
Not to diminish the importance of theological truths, but there can only be one church in the sight of God. I hope to see the progressive unification of Christians in my lifetime.
a) out of sight, out of mind
b) there is no Protestant tradition of opposing Eastern Orthodox or Oriental Orthodox teachings
c) Roman Catholics are the largest target, and therefor the easiest to attack
They attack the Catholic Church because for one thing we are the true Church. They however broke away from the Church which Christ Jesus founded and to justify their own beliefs they condemn the Catholic Church. The birth of Protestantism began the birth of subjectives, in which each man made Church interprets what they call the authority of their Church differently from one another. The result is heretical errors which of course is what happens when you remove Apostolic Tradition.
This is also why they ignore the Orthodox Church because condemning them would not justify their own beliefs. For starters they did not brake away from them so they are not a threat to their faith.