Sorry, but how are you going to pacify a country if your troops stay protected inside heavily guarded walls? (And once in a while get out, armed like the Empire troops in Star Wars).
A nation building policy, like the UN (and Sergio de Mello) has, requires opening up to the Iraqis.
The UN wants to return political power to the Iraqis.
All this, knowing the UN did not agree with this war (10 out of 15 countries in the Security Council were against)
Meanwhile, Bremer doesn't make that move (giving back to Iraqis) easier.

How can americans blame the UN when the whole mess was started by the US itself, without UN's approval, and yet, the UN did send people with a commitment to rebuild the country?

The US doesn't have a nice role in this game, and it's hard to face it; so, blaming others becomes the answer.

Raoul