In Tides of Darkness, she was admittedly fearful of Alliance retaliation. She knows she can't stand up against the Humans, Dwarves, and Worgen should they launch an attack at once.
Sylvanas also, by the nature of her people, can't reproduce soldiers as fast. In Siege of Orgrimmar, all factions on both sides suffered casualties and Sylvanas didn't exactly have a grand army to begin with after the Worgen began to turn many Alliance in the Gilneas/Silverpine battles into Worgen to prevent them from being raised by the Val'kyr. The Humans, Dwarves, and Gnomes have a much more easily replenished military, only having to wait a bit of time for recruits and not force battles to resurrect the dead. The Forsaken's population has been dwindling over time.
And if Sylvanas ever went full conquering mode, the Argent Crusade and Ebon Blade would join against her so fast that she wouldn't stand a chance. They're already watching her carefully after Gilneas.