Because chronology matters? Other than stating that Mists of Pandaria will focus much more on the fight between the Horde and the Alliance, we haven’t detailed any specific story elements of that fight. The attack on Theramore is an event which will take place prior to the release of the new expansion to aggressively advance the war between the factions following Deathwing’s death, further highlighting what Garrosh’s motivations are in contrast to Varian’s.

Garrosh wants the Horde to control all of Kalimdor. His attacks thus far have been toward that end, but Theramore remains a major port town and supply line for the Alliance. It’s an obvious strategic move for a power-hungry and prideful ruler such as Garrosh to seize Theramore.