Does anybody know why that the Church is promoting ecumenism so much? or the Theological theory behind it?
At the risk of being ignored, I'll tell you about the lecture that I had in adult formation class. There are two errors in the theology of ecumenism which distort the mission of the church. One is polytheism and the other is henotheism.
Many in this thread are rightfully rejecting polytheistic ecumenism. This type of ecumenism leads to indifferentism. The dreaded "I'm ok, you're ok" babble. Polytheism sees lots of different gods, but all being more or less equal, and we need to just get along with them to be ok.

mama mia, do I need to tell anyone why this is wrong?
But, the other error, henotheism, is more subtle, but it is an error nevertheless. Henotheism appears to be true in that its adherants purport to teach worship of the one true God, but they inadvertantly acknowledge the existence of other lesser gods. Henotheists want to call the world to monotheism, but end up promoting polytheism in practice. While they appear to do good to lead others to the true God, their attempts at leading others away from false gods actually gives a life to these false gods. Every unchurched people then has its own god.
Catholic ecumenism on the other hand is based on the theology of "radical monotheism". The teaching that there is only one God, the Father almighty, Creator of heaven and Earth. When others are evangelized, Catholics do not attempt to take them away from false gods, because the false gods do not exist. Catholics look at others in their attempt to worship the true God, but with imperfect understanding.
Our seperated brethren do not worship a false god, but lack perfect understanding of the Catholic faith. Moslems do not worship a god that does not exist, but their understanding of God is distorted. They atempt to worship the true God, but remain in ignorance of His true nature. The same with any other religion. Catholics should look for similarities in order to promote dialogue, so that understanding may increase. We do this because we are the light of the world, the salt of the earth, and a city on a hill which cannot be hid.
When the Pope kissed the Koran, he was not leading us into apostacy, but attempting dialogue and understanding with moslems, because there is only ONE God, not multiple. The same purpose is there with any ecumenical work. We want to promote the one God that exists, not give credence to gods that don't. We do this through dialogue and understanding, we are ambassadors of Christ.
That is why I believe in Ecumenism and I love the Catholic church
