The Red Sea crisis zone

Not an original point, but watching John McCain tie himself in knots trying to deny that oil is the reason that the US invaded Iraq brings up the obvious counterpoints: if the reason that the US keeps ground forces in Iraq is to prevent al Qaeda from taking control of what would otherwise be a failed state, then why are US troops not in Somalia and Yemen, where al-Qaeda linked insurgencies do threaten weak or failed states? Or, if it's sufficient to use air attacks to keep al Qaeda in check in Yemen and Somalia, why is that not sufficient in Iraq?