Because Saddam Hussein was a thorn in the side of the USA and Israel.
And GW Bush was against Iraq's oil being under the control of such a dangerous man, who had so often shown total disregard for peace in the region, as he attacked his neighbours one by one.
Osama bin Laden was not in Pakistan then, he was in Afghanistan and the "war against terror" started there first on October 7th 2001.
US troops were hot on the heels of Bin Laden and he narrowly escaped. But this was because Bush deliberately stopped the military when they got too near to him.
Had they got Bin Laden then, Bush could never have started the Iraq war two years later.
That the US allowed the Taliban to freely leave Kabul with all their weapons after completely surrounding that city, showed the political agenda of the Bush administration.
Bush could not start this Iraq war immediately, because they had not yet made up all the lies, and invented all the "evidence" which they needed to con the US public and the UN, into accepting this war.
And his administration's lawyers had not yet written all the legal documents which would provide lifelong immunity from prosecution for any of their deeds, no matter which parts of the US constitution or the Geneva Convention they breached.
That is how come !
For a number of reasons
1. Iraq is a oil rich country an bush saw it as a way to take over
2. Personal revenge, he made a comment stating he was trying to kill my dad
3. Democracy in the middle east
It's the oil m'boy, the riches it holds.
Ooooops ?
It doesn't make any sense? How come?