If you only quest Horde, you never see an unequivocal loss at the hands of the Alliance. Ashenvale's gains are reversed after you leave for Stonetalon. At the south of Durotar you can't prevent the humans from landing, but who knows if the landing party can even do all that well after the subsequent attacks on Northwatch. Southern Barrens is a tie, in a sense, but (UNWARRANTED SPECULATION TIME) the Horde is more likely to "win" in a sense by wearing down Twinbraid's forces or at least keeping them holed up someplace, even if a decisive victory cannot be gained due to lack of numbers. Swamp of Sorrows has the Alliance forces be fought off, with no indication of missing supplies or any of that sort. Stonetalon is pretty much Garrosh saying the victory was tainted and killing off his forces, or somesuch.

Alliance side, you need to look at Ashenvale with a magnifying glass and a lice comb in order to figure out how you've won. In Stonetalon you might be led to believe that the forces north may be defeated due to the Horde supply line being cut off by the victory with the Grimtotem's help. Swamp of Sorrows is supposedly a win, but Stonard is not changed in any way. Vashj'ir's Sandy Beach is clear of Horde (beyond the flight master and his siege weapon), so I guess that's a contested Stormwind victory? Felwood is an Alliance victory, but apparently the worgen screwed things up far worse than the goblins would have done. Gilneas is a contested victory, seemingly permanently due to the BG.

Also? The Horde is stronger this time around. They have better technology than the Alliance, and can easily catch up to any and all Alliance technological developments. Val'kyr act as an "I win" button, even if one named one was killed, due to the seemingly unquestionable loyalty displayed by raised Alliance characters. Some may say it's game mechanics, and I say that's bullshit because Blizzard could have easily shown a risen soldier running off, or trying to help the Alliance forces only to get killed by the Forsaken. And even if you discount the val'kyr's military advantages, they still compensate for the one Forsaken weakness (inability to restore numbers) that still matters. Magically, the Horde has the holy-arcane Sunwell, which, though limited in geographic scope, is still a very big boon.

And this isn't going into the Alliance's weaknesses, like suffering from crippling debt (whereas Orgrimmar had to be paid for thrice within a decade), disinterested elite forces (Tirion forgot to return the paladins after Wrath, apparently), night elves used as the Horde's punching bag (they never invested as much into the Outland/Northrend campaigns as the orcs, and so shouldn't have suffered as much; and of course the orcs fight in forests better than the night elves), the majority of worgen shunted into a useless semi-feral state, the Gnomish airforce being good for nothing, the Alliance navy constantly losing battles, and of course the "so farseeing they can't see they're walking off a cliff" draenei....

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1) You can't win.
2) You can't break even.
3) You can't leave the game.