Because if they connected the dots at this point, they'd have to explain why we are still playing footsie with Saddam over a year after he killed 3,000 Americans. And that would involve coming clean about the anthrax letters, and what they really mean, and the true nature of "Operation Amerithrax." And that would not be a happy state of affairs, would it?
They say that diplomacy is the fine art of saying "Nice doggie" until you can find a big enough rock. My hunch is that they have known since Day One that Iraq was behind the anthrax, but until they had the antibiotics stockpiled and the smallpox vaccines ready (in case he has that, too) it would have put the entire country at risk to move against him. Plus they needed time to come up with a plan -- and move the resources into place -- to hit so hard, so fast, that none of that stuff gets out of there while the battles are going on.
This is a very tricky thing they are going to attempt. People here could die in this war. But... it was coming sooner or later. Better now than when he has more of it, or more of it pre-positioned here.