That's the hard question to answer. The best I can explain is that the guru's announcement is necessary to create the base case for the induction.
Imagine the 4-person scenario with 2 blues and 2 browns. Is it common knowledge that blue-eyed people exist? You might think, "Of course, because each blue-eyed person can see 1 other blue eyed person!" But keep in mind that nobody knows their own eye color. Blue-eyed person A doesn't know their own eye color, so he doesn't know that blue-eyed person B can see A's blue eyes. If blue-eyed A believes he has brown eyes, then he knows that blue-eyed B won't see any blue eyes! The fact that blue-eyed people exist is not common knowledge!
Add third blue-eyed person C. You'd think the above paragraph falls apart, but it doesn't. C knows that A sees B's blue eyes, sure. But C doesn't know that B knows that A knows that C has blue eyes, because C doesn't know his eye color! The idea that there are at least two people with blue eyes is not actually common knowledge, because again, nobody knows their own eye color. In addition, the sentence "A knows that B knows that C knows there's at least 1 person with blue eyes" isn't actually true, though it may seem that way at first glance. For it to be true, A would have to assume that he has blue eyes, because he's the one doing the presumption. B knows that C knows there's at least 1 person with blue eyes because B knows that C sees A. But A doesn't know the previous statement because he doesn't see A, because it's himself!
Imagine the 4-person scenario with 2 blues and 2 browns. Is it common knowledge that blue-eyed people exist? You might think, "Of course, because each blue-eyed person can see 1 other blue eyed person!" But keep in mind that nobody knows their own eye color. Blue-eyed person A doesn't know their own eye color, so he doesn't know that blue-eyed person B can see A's blue eyes. If blue-eyed A believes he has brown eyes, then he knows that blue-eyed B won't see any blue eyes! The fact that blue-eyed people exist is not common knowledge!
Add third blue-eyed person C. You'd think the above paragraph falls apart, but it doesn't. C knows that A sees B's blue eyes, sure. But C doesn't know that B knows that A knows that C has blue eyes, because C doesn't know his eye color! The idea that there are at least two people with blue eyes is not actually common knowledge, because again, nobody knows their own eye color. In addition, the sentence "A knows that B knows that C knows there's at least 1 person with blue eyes" isn't actually true, though it may seem that way at first glance. For it to be true, A would have to assume that he has blue eyes, because he's the one doing the presumption. B knows that C knows there's at least 1 person with blue eyes because B knows that C sees A. But A doesn't know the previous statement because he doesn't see A, because it's himself!
It's certainly hard to wrap your head around. There's other explanations at http://forums.xkcd.com/viewtopic.php?t=80149