The short answer to why the US didn't find Weapons of Mass Destruction in Iraq is, of course, that there were no WMD in Iraq. The search had an unlimited budget and it turned up zip.
The further question is, given the enormous political stakes, both for the Bush administration and US imperialism as a whole, why no WMD were planted in Iraq, when it became clear that the search would not pan out.
Anyone familiar with my writing already knows that my answer does not rely on the integrity either of the Pentagon or the Bush Administration.
I think the answer is that things like that really aren't all that easy to fake and the political consequence of being caught in that lie would have been much worst than finding no WMD at all.
In the first place any "found" WMD has to have a creditable "legend." It had to be made somewhere with materials obtained by Iraq and manufactured by people in Iraq and that legend makes it much more difficult than simply planting a load of poison in a warehouse because it will be subjected to exhaustive tests also, not just the WMD. And then there are so many people that would have to be in the loop, the chances are great that somebody would spill the beans.
I only bring this up because many people on the left seem to favor elaborate conspiracy theories to explain everything from the Arab Spring and the Libyan Revolution to the collapse of the twin towers. If ever a situation cried out for the people in power to put the fix in, it was the failure of the Bush administration to find WMD in Iraq.
Just a cautionary tale that came to mind while watching the Green Zone...