At this point, there are only two possibilities I can think of as her reason. 1. She was too lazy to manage such a huge territory. As she's just said before, she only wanted to be the lord of Barren Land. She didn't want to be (female) emperor and such. Her position as an overlord above the four countries rulers is already very good. She got them under her thumb, but she doesn't have to manage their territory. 2. She do it for the overall situation of the war. But, I wouldn't guess what it was. After all, I feel like the characters settings here (and sometimes the mentioned plot too) can be changed at will without rhyme or reason. I give up.

Jun Wu Xie just simply got rid of the emperor...

This way, even if they don't have enough time, LY can still try to train the army and make sure they get proper weapons and provisions. But if his father doesn't step down, not only will he do fuck all, he will probably run when the war starts.

Ah, figured :') Now, do we lock up the emperor or kill him? Haha ?

Its just befitting to abdicate since your not a competent emperor might as well step down and just grow mushrooms..