Because Yin Lian is Yu Mo's most genuine captive.
They have never won against Yu Mo, and since the two have fought many times, Jiang Si naturally remembers it.
Yu Mo and Yin Lian became Magical Girls around the same time.
Yu Mo almost joined the Disaster Strategy Bureau, and as Magical Girls of the same period, their realms didn't differ too much.
For a certain period, Yu Mo had some difficulty dealing with Yin Lian, and Yin Lian certainly had chances to win.
But as time went on, the gap widened, until later when Yin Lian was basically thoroughly beaten.
Don't be fooled by Yu Mo's timid and easily shy appearance; in reality, she's very competitive.
As the Sect Leader, Jiang Si highly appreciates such a diligent disciple, who usually secretly follows him and works hard.
Even if Yin Lian caught up just a bit, Yu Mo would double her efforts, fearing her strength couldn't surpass Yin Lian.
