One of the primary purposes of that Council was reunion, so naturally the Church wanted their participation. The Eastern Orthodox have had representatives at other Ecumenical Councils as well, such as at Vatican II.
This brings up an interesting discussion – that may deserve its own thread:
What amount of representation is needed in an Ecumenical Council. Certainly the ideal would be that every Bishop in the world be present – but this, as far as I know, has rarely been realized in history. Also, what about our separated brethren? For example, after the Council of Chalcedon, few if any Oriental Orthodox participated in the subsequent Councils of the first millenium, but both Catholic and Eastern Orthodox admit to their Ecumenical nature. Thus wouldn’t it be natural for the Catholic Church, since it holds that the Eastern Orthodox are sadly separated from Catholic Communion, to not see it as necessary for the Eastern Orthodox to participate in the Councils for them to be Ecumenical?