actually it's cool I think I have it now
We bombed our way into the place for political/strategic reasons. An ideal opportunity to take out an erratic regime, solidify our hold on the middle east, and as an added bonus get a grip on a major oil producer, right after we suffered the worst attack on US soil since the Battle of Antietam at the hands of a radical islamic group who we could prove had met with Saddam (even if we had no proof he actually supported them). And to top it all off this was a country led by a dictator who had used chemical weapons on his citizens. And who we had vague, half-substantiated reports about him trying to develop nukes. The whole thing was a neocon's wet dream. An absolute slam dunk in their minds, which is why Cheny and Rumsfeld were all over it.
Oil was part of it, but it wasn't the primary reason. More of an added bonus than anything else.
Not just a "wee mistake". A horrible, massive one, that led to so many unnecessary deaths.
EDIT: Also "cheap" shale oil is way more expensive to extract than crude oil.