State Of Things.

Ghouls can be killed with the right tools. Death by concept within an Origin domain will kill them. The problem is that it is difficult to do if the ghouls are in the millions. Then the Origin god would be killed in their own domain. That's why ghouls would target Origin gods when Primogenitors aren't around. 

Origin gods are above normal vampires on their attack priority because while Origin gods aren't the cause of the problem, they are a threat to their mission of getting rid of the Vampires. These Origin gods did interfere with the mission of the ghouls. That particular endeavor cut down their number within the plane from 41 to 31. 

Then the Vampires betrayed them and used them to create more Monarchs. It reduced their number from 31 to 14 and increased the number of Monarchs from 15 to 30. That particular event turned the tide of power irreversibly away from them.