Blowback is the best resource for the answer to this question but in short

1. Kuwait was purposely overproducing oil to undermine the Iraqi economy. The Kuwaiti contribution to the war effort ended up costing about $32 billion USD, Iraq had only requested renumeration of $10 billion prior to the invasion. The conflict was never about money or democracy it was about destroying Iraq.

2. The US government turned Iraq into the state it was in support of them during the Iran-Iraq conflict (including making it the fourth largest military in the world in 1990), and US diplomats expressed at minimum that they had no position on inter-Arab conflict (many have interpreted US statements as tacit endorsement of the Iraqi invasion).

3. The US invasion was in defense of an autocratic petro-state to prevent an emerging successful Arab Republic and to keep most of the Middle East crushed under the boot of imperialism. The bloodshed (including numerous war crimes) and subsequent sanctions that occurred due to US intervention were far worse than anything that might have happened had they not intervend. Furthermore the American public was extensively lied to about the invasion, such as the Kuwaiti ambassador's daughter's lie that she told under oath before Congress that she had witnessed Iraqi soldiers killing babies in incubators. While the lies alone don't justify Iraq's invasion, it should raise the question why Americans had to be lied to in order to gain their support. All the lies about democracy and freedom fall hilariously flat faced when you remember Kuwait is literally a brutal autocratic monarchy.