Because all of the Demon Tribes were warring with each other in some way, even when their Council functioned at 100%, neither single of them had the capacity to fight one of the Gates because they needed to leave a considerable part of their own forces, to watch for other Demon Tribe or Species that would try to profit on their situations and attack them while they were off.
"Indeed, though if the rumors are to be believed, then the Star-Moon Coalition has been doing what it could to usurp part of the market on their side. Especially, they have been investing a lot of effort into a slave business, probably aiming to cut the income of the Golden-Horned Demon Tribe considerably, so their wealthy status may shrink in the near future."
