The players were not only reinforcing the wall and making it stronger, but they were also adding some of their own things to it. Large towers were being erected by some players while others were making turrets that would be placed on the towers, as well as smaller ones for the wall.
The demons had placed some form of turrets on the wall, but they were weak, unable to deal any significant damage to the monsters. It wasn't as if the demons were unable to reinforce the wall more or place some arrays and defensive turrets on it, they simply chose not to.
As for why, well, it was because the wall was the one thing that all of them were involved in. It was built thanks to the cooperation between all the tribes, and none of them wanted to waste any more funds or materials to further reinforce it, not to mention that they disliked placing turrets since that would mean that the weaker demons could potentially kill a monster and take the exp, which they resented thinking about.
