The Black Lotus Flower did not choose Wenren Xuan as its Tribulation Master.
The so-called Tribulation Master is not the master of the Black Lotus Flower, but the servant chosen by the Black Lotus Flower to help spread its disaster.
In return, the Tribulation Master can tap into the power of the Black Lotus.
As for Wenren Xuan, he was nothing more than a disposable tool.
The real goal was to have him kill Long Yan, while letting the Ghost Race in was a farce.
How could the Heavenly Ghosts of the Ghost Race easily come to this world? They fear death as well, so at most they would send their minions to scout the way, and only when things were stable would they descend to make trouble.
Otherwise, what if they were intercepted by Celestials when crossing the barrier? What would they do?