This question was answered by Caydiem, Community Manager, and it makes sense. Blood Elves are the new playable race announced at BlizzCon to come included within World of Warcraft: The Burning Crusade Expansion:
Caydiem: “Chris Metzen was asked about this very thing in the Lore and Quest Design panel during BlizzCon. This is indeed part of the history, and it hasn’t been forgotten, but ask yourself a similar question: “Why would Tauren ally with the Undead?” That’s a tolerance more than a true “alliance”. The same holds true here.
The bonds keeping the Blood Elves to the Horde are not ones of fellowship. The Horde is the means to an end, a particular purpose that the Alliance has actively fought against in the past. There is some bad blood there, no one denies that, but only through working with the Horde can they access Outland and gain access to the tremendous power there.”