I Would guess that it is because it is a different sorta class (like healer) and if you have already gotten "Weapons Expert" that would make you melee class so then having both that and "The Low Priestess" might be a bit overpowered as you would be able to fight as melee class but also heal your companions.
I believe that those two skills are new class and if you want to play with those skills restarting will be needed just like how if you want to play with the mana knowledge skill you would also have to restart.