Perhaps because of their bloodlust, Splatt. In opposite of the more peaceful High Elves, the Blood Elves were more desperate to find their precious "magic."
But also, not only did the High Elves got a suprise attack from the Scourge, they also didn't expect any combat at all. It's like real life, it is obviously easier to suprise attack a big city, instead of attacking a city that already got enough time to prepare their defenses.
Of course, this is my opinion of the difference between the Blood and High Elves. I, for one, prefer High Elves.
Oh wait, this isn't what the topic is about

[/b]