During Ryu's first life, Mental Realm cultivation was more than just taboo, it was practically banned. It was to the point where even the Heir of the Wind Shrine, Gale, despite having the Seventh Ranked Heavenly Pupils which greatly boosted one's Mental Realm ability, still had to be cautious despite his background.
Yet, during this time, Necromancers didn't face the same banishment. It was no wonder Mental Realm Masters felt unresigned. After all, no matter which branch of Necromancy you practiced, what was most important was your Mental Realm. But, somehow, Necromancers thrived during this and the last era, while Mental Realm Masters became cockroaches pulling themselves back up by their own efforts.
Of course, this was simply a one-sided perspective. Human beings had a habit of seeing the flaws in others without doing any self-reflection. Could it be that the Shrine Plane was truly biased? No.