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.”

On another post, a fan refered to Thrall, Vol’jin and Cairne as retards, to which Caydiem answered:

Caydiem: “Say there was a new, powerful force in town (and I’m referring to both the Forsaken and the Blood Elves here, in their respective times) with the potential to be either ally or enemy… and you’d only recently re-established your people on a new land and recovered from endless invasions. You’re vulnerable, and increasingly aware of your known enemy gaining in power with each passing day. What do you do with this new force? Ignore it, allowing others to potentially gain even more strength against you? Attack them, making them yet another enemy? Or do you form a tentative and closely-watched alliance with them, bolstering your strength and giving you access to things you didn’t prior while being able to keep a careful eye on them?

Thrall’s no fool. Allying with the Forsaken gave his people a foothold on the Eastern Kingdoms, and he can keep a close eye on any potential… problems they may cause in the future. The Blood Elves are a strong, sophisticated, arcane race with knowledge of Outland as it is now rather than Draenor that was—to go in blindly to new frontiers would be folly.

Do not so quickly dismiss the choices of Thrall, my friend. He may initially seem quite the trusting fool, but he is deeply entrenched in the politics and power plays of Azeroth.”

I want to take the opportunity, now that you are reading.  Sometime during the week, Blizzplanet might be a couple of days offline.  We will give you the scope of why after the site is back.  We are making a bold move to support Azzor’s NoGold Stand.  Confused?  We will tell you when you see us back online.