Sylvanas has been listed as the leader of the Forsaken since WoW Launched.
Show me where Bolvar has been included in ANY starting cinematic or marketing material. He was as much Alliance as Gamon was Horde.
In other words no where near the same level. Also, Maybe if Sylvanas disappeared in the 3rd expansion and never came back, you would have a point. Or if the entire last expansion was about Bolvar leading the Alliance, you might have a point.