to the question of :
why aren't the chemical weapons Saddam used on the Kurds, and the chemical weapons he wanted to use on the Kurds again, ever used as proof of him having WMD? Certainly the chemical weapons that wiped out over 100,000 Kurds would be considered WMD, right?
this is the answer i can offer:
Because they weren't there. Chemical weapons do not store easily; they deteriorate over time. Moreover, they're difficult to weaponize. So if you've got them, you have to use them quickly, and carefully. The question that the Iraq War is premised on is was Saddam Hussein reconstituting a WMD program?
Reconstituting is the operative word, because following the first gulf war his chemical/biological/nuclear programs were destroyed or dismantled. For most people, the inspections in the ensuing years provided enough proof that they remained dormant.
Plans for the current Iraq War were hatched in 1996. Richard Perle, who had been President Reagan's Assistant Secretary of Defense, completed work with a study group (for Israel's then-Prime Minister Benjamin Netanyahu) concluding that the Oslo Accords could not guarantee security for Israel. The study also concluded the accord should be thrown out, and replaced with a strategy for regime change in Iraq, Syria and Saudi Arabia. Those who subscribed to this notion became known as the Clean Break team. Part of what drove the Clean Break thinking was concern that a mistake made prior to the Gulf War could be repeated.
Prior to that war (which was an international effort to enforce a United Nations resolution calling for nothing more than for Iraq to be expelled from Kuwait) the US Intelligence Community was pretty sure Iraq was at least 3 years away, and maybe as many as 5, from producing a nuclear weapon. After the war, an alternate program was found, which was only about 9 months away from that objective. Even though the consensus was that the UN Inspections were containing Saddam's programs, nobody wanted to be caught underestimating again; when inspectors were refused access to certain areas, the Clean Break people saw their intelligence as fragmentary, incomplete, and assumed the worst (inspectors were probably denied access to these areas because Saddam needed to protect his stature, and his regime, in the Arab world).
President Clinton was actually close to authorizing an overthrow in Iraq. He had General Anthony Zinni put together a detailed plan involving large numbers of troops and a two front campaign (troops staging from Turkey and Kuwait). The plan was shelved in favor of smaller, more covert actions after the Lewinsky affair became public. This was the end of the Clean Break plans, for all practical purposes, until 9/11.
After 9/11, the Clean Break advocates were joined by people like Condoleeza Rice, who was concerned about Iraq's relationship with terrorists plotting against the United States (Zarqawi had received medical care in Baghdad, and it was thought unlikely that Hussein was unaware of his ambitions, even if he was not assisting him). Donald Rumsfled, incidentally, ended up feeling that the Zinni plan called for far too many troops, and the force that eventually invaded was substantially smaller than first envisioned.