That is because it wasn't necessary! The invasion of Iraq was preceded by a fully engaged debate in Congress, and the airing of the available facts on Iraq. It resulted in the Congress PASSING a resolution to Authorize Use Military For in Iraq - with large bipartisan support.
There were a few conservatives who felt that the 23 "whereas" reasons for AUMFI were insufficient for deposing Saddam Hussein - but there was no disagreement on the facts of the 23 'whereas' statements. Honest people can disagree.
But many Democrats who VOTED FOR the AUMFI - when things started getting tough in Iraq - turned and aided/abetted the enemy by pretending that the 'invasion' was ONLY about WMDs - pretending the AUMFI and 23 whereas reasons didn't exist. Those Democrats gave succor and comfort to terrorists to attack our military service members. Like JF'nKerry - who was FOR the AUMFI before he was against IT - they LIED to America about what Bush said. Those limp-wristed liberals deserve the real condemnation for politicizing and LYING about what happened in Iraq.