Because it's pretty much false. I'm not saying for certain that nobody tried to DDoS Blizzard at all, but it's not a significant part of the problem if it even took place. These are not the signs of DDoSing. When a game gets attacked like that, everything is fucked and you lag everywhere. This is clearly just overcrowding and insufficient load-spreading. The low pop servers have been fine, as has any zone that isn't overcrowded.
The DDoS excuse was just that: an excuse meant to shut up some of the complaints. I'm not convinced there has been any major DDoS attack at all, aside from the fact that so many players trying to login/play at once is effectively a DDoS in its own right. No hacking group has been trying to take down WoW, at least.