I don't think popularity is the main difference between these BLs. Thriller Trainee is a Chinese webnovel, while this is a Korean webnovel adaptation. Korean artists/companies might just see a bigger market for horror BL as opposed to their Chinese counterparts. There might be other logistics with the adaptation that companies just don't want to deal with. You probably will have to wait for a Chinese company to come around to get a Thriller Trainee adaptation
Yeah I know Chinese companies are a little less developed compared to Korean ones. But it is still surprising when Chinese novels are so insanely popular. I'm also surprised that even less known Korean novels get adapted before literarally the best and most popular of the same genre that are Chinese.
True
Why are the more popular BL horrors not getting adapted???? This looks good, don’t get me wrong but why are we not getting for example Thriller Trainee