The only conclusion that I can come up with is that the planning didn't include any contingency planning: as in, "what happens if there is no rebellion against Hussein by the population at large?"
Considering the noises coming out of the career military folks, I suspect that question got asked, but was dismissed by the People In Charge.
The right way to use a military is to say "here's the goal; figure out how to do it. Keep me briefed." I don't think that's how it happened.
This is the kind of thing that happens when decisions are driven by ideology rather than actual pragmatism.