Originally Posted by
Hallower
Pit Lords aren't summoned as easily as Felguards and Doomlords. Pit Lords require a portal and a LOT of juice to power it.
Fel Orcs are "fel" for a reason, they're controlled by the legion and thus part of their ranks.
Ogres and Gronns in the Twilight Cult are easy to explain: Old Gods. As cliche as they're becoming if an Old God has control over Deathwing, it's child's play to get and maintain control over Ogres and Gronns.