Given the circumstances of the upcoming confrontation, Melkor decided to lay an ambush.
After all, why try to set up a spell array in the middle of a battle while phasefighters were shooting the mechs that carried the crucial formation anchors?
The chance that something would go wrong was too great!
One of the first lessons that a formation master needed to learn was that when one wanted to employ a spell array proactively against an enemy, it was best to employ them as traps.
By setting a trap in advance, the formation master would be able to take the time to calculate the right placements without any urgent time pressure.
Another factor that played in his favor was that the absence of fighting meant that the chosen battlefield remained relatively static.
If energies started to spew in every direction while asteroids were blowing up, Melkor may be forced to redo his calculations as his previous ones were no longer accurate anymore.