Originally Posted by
Caerule
I always assumed that Sargeras doesn't know our physical location in the Great Dark Beyond.
However the book states he's just very far away on his crusade to burn planets, and that it would take him ages to get here. Hence trying invasions by Portal.
Travelling by spirit may be a lot faster, as the Titans' Spirits accomplished this, in seemingly short time. But that wasn't very successful.
Honestly I still like the explanation that Sargeras doesn't really know where Azeroth physically is in the Beyond, only knowing where the Twisting Nether connects to it, thanks to the Highborne. But the Chronicle book states that Sargeras is simply too far away.