Earlier on in the epic Satan had battled with Michael, a general in Gods army of angels and lost. Satan was maimed in the battle and came away with his right side cut off by Michael's magical sword. Satan has learned his lesson and will not make the same mistake again by engaging in hand-to-hand combat. Satan with his cannon scheme will be able to fire at the angels of Heaven from a distance and do damage without taking any casualties. Again this is another way for Milton to show the awesome power of Satan's new weapon, that it can still do so much damage from such a distance.
This is obviously the smartest move for Satan to make in furthering battle with God and the angels, but Milton is going further with this. Satan has always languished in his pride and now he has smartened up and put pride aside to be humbled by his inferiority to God. Satan is afraid, but he is resourceful.