Quote:Because they have frosaken the light (or feel they were forsaken by the light). It's part of the reason for the name.
The Blood Elves have also forsaken the Light. Kael'thas brought back a being of Pure Light and they're enslaving it to teach BEs about being Holy. Problems with that aside, those holy capabilities were apparently something the Horde felt to be strategically important enough to accept the Blood Elves. It's not clear why they didn't make the BEs send trainers to Deathknell to pump out some Forsaken Blood Knights as part of this deal--Oh wait, yes it is clear! Aspiring Hordadins have to buy the expansion if only Blood Elves can be Pallies.
My bitter lore commentary aside, this is a necessary step for Blizzard's overall game development strategy. They'll want the new races to be really attractive to get people leveling together and building social ties all over again. It helps draw people into re-using all the existing content if they're leveling new characters. I'm sure level 60-65s will be doing pickup ZG/AQ20/etc. To encourage this, they're piling incentives on the new races and this is just one of those incentives.
It also lets them cut out half of their in-house raid play-testing. That way, they can push raid content faster and cheaper.