There's a few things. At the moment, Bolvar doesn't seem as powerful as Ner'zhul or Arthas were. He's able to keep the undead on Northrend dormant, but it doesn't seem like he has much control over the Scourge outside of the continent. There's a pretty strong Scourge presence in the Plaguelands still, with the Cult of the Damned still active as well. The Scourge is also present on Kalimdor, still controling Razorfen Downs.