The 1990 sanctions on Iraq were put there 6 months before the us started its bombing in january 91. Sanctions need time to work. You don't feed poison to a person you plan on shooting, you shoot them after the poison doesn't work.

It could be that this is just a symbolic gesture for the people within the us ruling class. a sort of "we can all agree on sanctions so lets do that while we continue to debate whether we will invade or not" But even that shows that they know they have to do something but there is still little support for to war directly. Its the "no invasion" side throwing a bone to the rabid zionists.