The Explorers
Upon checking the abilities and powers of the best builders and mechanics the territory had, Reign and the others decided to postpone the creation of the gate. For now, what the players were able to create was a bit below what Reign and the others wanted. The gate had to be strong enough to defend against even high-grade Tier IV monsters as after some time passed, the fog would definitely spread to the wall, and more powerful monsters would also start appearing.
It wasn't as if the fog couldn't send out the monsters outside, it was just that it chose not to do so right now. Sending the monsters outside meant that the control it had over them would be lowered, and the power of the monsters would slowly start to degrade, until eventually fading away.