It is likely that the reason is that the consular official reasonably believes that you committed a material misrepresentation of fact. That is, you lied in a way that causes a permanent bar to admission into the U.S. The only option is a waiver. It is unclear whether the consular officials will recommend approval of the waiver and what it will take to qualify for a waiver in the consular officials discretion.
You should schedule an appointment with a very experienced immigration and visa attorney. You and your fiance will have to consider whether to go through the waiver process. This is unfortunate. This is why applicants should hire experienced immigration attorneys for the entire visa process.
The above is general information, not legal advice, and does not create an attorney-client relationship.