Why do some people believe it was a mistake for the united nations and the united states not to occupy iraq after the persian gulf war?

Some people believe it was a mistake for the united nations and the united states not to occupy iraq after the persian gulf war because of simple outcome that regime of saddam would be overthrown by then and due to lack of developed radical islamic movement the stabilization of iraq wouldn't be such problematic and brutal as it happened after 2003 invasion.