Two ways to look at it.
Either the two "former" enemies have set aside their differences and come together to drive us out of their territory or the reason we see them together now is because there is now a common physical ground on which to fight us that didn't exist before the war.
Just because Saddam's regime is not in power any longer does not mean they are "gone". Your use of the word "insurgents" conjures images of peasant farmers taking up arms. It seems likely that we are still fighting Saddam's regime and his ghost army that dissolved into the civilian population... the same outspoken enemies of OBL.