As with anything complex there will be a lot of reasons. I think Saddams decision to sell oil denominated in Euros was probably influential. https://www.theguardian.com/business/2003/feb/16/iraq.theeur...

Primarily though I think the US govt at the time genuinely thought they could do regime change and create stable democracies in both Afghanistan and Iraq. They doubtless had many murky reasons for wanting to believe they could do this, but I think the belief that they _could_ do it was sincere if proven false by events.