He'd been doing that off and on since the first Gulf war. What made it so important to invade in 2003?
The lesson of 9/11 was that weakness in the face of attacks is provocative and will inevitably lead to greater and greater attacks. Bin Laden saw that we weren't responding to Hussein's overt attacks in a meaningful way (e.g. Operation Desert Fox in 1998 was a joke) and recognized that as a failure of will on our part. So Bin Laden took our failure of will as a green light to escalate his attacks against us until the attacks were so large and so damaging that we couldn't ignore them any longer.
Apparently, the lesson of 9/11 is easily forgotten because now a lot of Americans, including many conservatives who should know better, wonder why couldn't have just ignored Hussein thousands of attacks against our pilots.