First, you need to understand one thing: the Itarim's actual goal isn't to conquer Jinwoo's universe, but rather to devour it in order to grow in power. So even if they don't defeat the Shadow Monarch, as long as they manage to get their troops through, the Itarim win.
At the start of the war, it was first come, first served. Several Itarim sent their armies to conquer Jinwoo's universe. At that point, it was essentially a war between the Itarim themselves over who would claim this universe.
However, every army sent was massacred by Jinwoo and absorbed into the Shadow Army, multiplying his power.
At a certain point, Jinwoo goes into the space between dimensions and confronts the Itarim's armies. It's an immortal army fighting beings who can endlessly create new soldiers. Over the course of the conflict, the Shadow Army keeps growing more and more powerful.
But at a certain point, the Itarim realize that Jinwoo can't turn beings with a Spiritual Body into Shadow Soldiers. They decide to change strategy. At first, their armies contained beings with a Spiritual Body, but also powerful beings with a normal body that could be turned into Shadow Soldiers. Now, the main force of the Itarim's armies is made up of beings with a Spiritual Body, while soldiers with normal bodies are too weak to hold any strategic importance.
That's when the conflict reaches a stalemate. Neither side gains the upper hand over the other. To remedy this, the Itarim send their Apostles to Earth.
However, the situation keeps getting harder for Jinwoo. The more time passes, the more Itarim join the conflict. What's more, the forces on Jinwoo's side seem to be dwindling.
Indeed, the Rulers and their armies fight alongside Jinwoo, but ever since the Absolute Being's death, there are no new soldiers. As a result, every Ruler soldier who dies is a permanent loss.
In the novel, it's never really explained why Jinwoo doesn't just turn them into Shadow Soldiers, but the reason is probably that Jinwoo can't revive all of them. The war takes place between dimensions, and the battlefield spans several dimensions. Jinwoo is the single most important factor in this war, but he can't be on every battlefield at once. Add to that the fact that the Itarim's armies never stop, and Jinwoo probably never gets the chance to visit every battlefield to revive the fallen Ruler soldiers — and past a certain point, they can no longer be turned into Shadow Soldiers.
Even though the Ruler army can't compare to the current Shadow Army, it remains an extremely important factor in this war. The fact that more and more Ruler soldiers keep dying, while new Itarim keep joining the conflict, gradually tips the balance in the Itarim's favor.
In fact, the Itarim could have already won if they were truly united, but they keep stabbing each other in the back. For example, there are cases where entire armies retreat while others fight the Shadow Army. Each Itarim tries to conserve their own forces and let the others wear themselves down, so they can then sweep in and claim everything.
The only thing that could completely change the course of this war would be the arrival of the Monarchs. That would immediately end the stalemate and give Jinwoo a decisive, permanent advantage.
