This was talked about back when I played and there was a number of reasons as to why it doesn't work on bosses. But the main one was simply to slow down soling a boss.
To my understanding from when I played everything up to Dragons abyss was soloable by a Taoist. But then they buffed some of the older bosses, to inspire grouping Taoists have always been a support class in Mir so them killing substantially slower than the other 3 is completely understandable as they have the least trouble finding a group for the bigger bosses.
Although things could have changed since i left so take what I've said with a pinch of salt.