There are 5? skill groups and only one skill can appear from a skill group. For example, if two skills are in skill group 1, they can't appear on the same equipment. Herding doesn't have a Powerscroll available. Just spot checking the list of Powerscrolls available, it doesn't appear many skills with PS aren't in a skill group. Some notable ones are the ones that are crafting.
I think it's frustrating too, but I think the issue goes deeper than this. I think 120 skills shouldn't have been introduced at all or that + skill attributes hadn't been and skill cap had been raised.