You don't really need lore though because it is as simple as "X race got interested and was trained". Even for Paladins and Warlocks. Paladins only require the Order to open their doors to teaching those races. A paladin can be cut off from using the order's magic but not from using the light. So there already is a lore basis for it being Order specific.
Warlocks can simply be outcasts learning to use warlock stuff after years of it being present in the factions. Even for someone like Dreanei it can simply be an outcast. There doesn't need to be any big lore reason just because. Would it be nice? Sure but look at gnome priests. Their lore was essentially after years of seeing it done they decided to learn it and experiment with light based technology to retake/heal their capital.