Longer lifespan means longer time to be corrupted. Plus, they don't have to worry about Black Ajah, the Three Oaths are pretty effective at ensuring any minions will do as their told (Verin notwithstanding). They can threaten a person with death or binding, and after the binding be pretty sure that the person will stay true to the cause, or at least not rat them out. You don't have that assurance with non-channeling people.

Plus, they're a lot of powerful people grouped together. Some join for the chance to get ahead as Aes Sedai (Sheriam).

And a Black Ajah is always a powerfully placed Darkfriend. If they're fairly low in the hierarchy in the Tower, they can go out into the world and rank near any High Noble, regardless of what country they are in. So the Shadow will concentrate on converting them more than other normal people.