The combined attack above the divine Lord realm needed a 'carrier' to set up, and it was very, very difficult to make a 'carrier'. Not only did it require precious materials, but it also needed the person who could make it.
It was much more difficult to make a carrier of a combined formation than to set up a formation alone or to make a divine weapon.
One had to have extremely high attainments in both runes and formations and in artifact-making in order to succeed.
Most importantly, many methods of creating the 'carriers' of joint attack formations had been lost.
Therefore, the destruction of the carrier of the joint attack formation was a huge loss.