When the US and Soviets destroyed long range bombers to comply with treaty obligations, video, satellite overflights, and humans witnessed the 'dismantling' of Bears and B-52s. It is easy to show compliance if you want to.
If Iraq got rid of WMD's all they had to show was the documentation. That would have been simple, and would have quickly taken the wind out of the sails of the military option.
Iraq did not show that they complied with the dozen or so UNSC resolutions that required Iraq's disarmament, and it is as simple as that.
It has only been a few weeks since the start of the campaign, and some pfc doesn't have the time to be looking for WMD's when he is keeping his eyes out for more immediate personal threats.
Once the dust settles a bit, and inspectors start looking in places that Hans Blix and UNSCOM weren't allowed, the true story will come out.
And the biggest Iraqi tactical mistake in the war was being led by someone who has never won a military conflict. (unless he was attacking his own populace)