Titles are given based on your classes' Class Hall, hence, it makes sense for warlocks to be Overlord, because you can interpret it that this title is because their Class Hall is on one of the Legion's homeworlds if I understood that well (like 'over' Azeroth, not from Azeroth, hence Overlord title...). Just to make analogy why Skylord fits warriors as well is because their Class Hall is in the skies. If titles weren't derived from a Class Hall philosophy I would have to agree that Overlord would fit warriors the most, but since they aren't it is the best as it is...