Irina provided a reason, although Gu Fan's method could only send two people to Dark City at a time, slowing down both speed and timing, and indeed Gu Fan had ample time to encounter the monsters of the Dark Night Primitive Forest, any injuries sustained there could affect subsequent actions.
Gu Fan also gave a reason, the monsters of the Dark Night Primitive Forest would always have to be confronted. As everyone entered Dark City in groups, there would surely be some who would encounter the monsters on the way and end up isolated and without support.
It was the same argument, unless there was a way to send everyone to Dark City at once, they either had to follow Gu Fan's method or everyone would split up.
After gritting her teeth, Irina made up her mind after a long while, "Actually, there is a way to send a lot of people to Dark City all at once. Your friends Jie, Hai, and several from the Hunter's Guild all entered Dark City at the same time."