News flash! Garrosh + his Kor'kron do not equal the Horde. The Alliance does not win anything-- they help overthrow Garrosh with the Horde. Furthermore, this raid is supposed to be a bloodbath and the Alliance is over-extended. A war between Alliance & Horde forces following Garrosh's end would not go well for either side. Hell, the only reason the Alliance is even involved in the first place is because Vol'jin asked for their help.