N'Zoth doesn't have much in the way of standing forces, though. You see in the Nya'lotha raid the majority of his military firepower, and you burn through it with a will on your way to his carapace. N'Zoth's overarching goal is to take over the Forge of Origination and the Titan systems stabilizing Azeroth and use those system to overwrite Azeroth's reality and change it such that it is, and always was, his own Black Empire. That is why his focus is on Uldum and the Vale - they're the locations of the Titan machinery that will make this possible (e.g. the Engine of Nalak'sha which is connected to Mogu'shan Palace in the Vale and the Forge of Origination in Uldum). Stormwind and Orgrimmar would be too defended for his forces to take at the current time, but if he succeeds in taking either Uldum or the Vale he needn't worry - the resulting overwriting event will make it such that both capitols are and were always his, and nearly no one will be left to challenge him.