Talent
While Xue Wei could bring Qi cultivation to the Ghost Realm, it was not as outstanding as one might expect.
To be able to use this Qi cultivation, one needed a Marrow-Cleansing Pill, or to have just created their spiritual bodies. The Marrow-Cleansing pills could not be created in the Ghost Realm, and those who had just recently become ghosts were also not capable of getting their hands on a Qi cultivation method.
So, even if the method got spread throughout the entire Ghost Realm, it was still impossible to train in it without the assistance of Xue Wei.
The middle-aged ghost was not willing to announce the cultivation method to the rest of the world as it would not do him any good.
The world was such that no one would be happy if others had something unattainable, and thus they would destroy it since they could not get it.