Hi, zmand97,
Sometimes a corrupt or missing preference can cause iMovie to act other than as expected.  Try deleting/resetting preferences and see if that solves the issue.  To delete/reset preferences open iMovie while
holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you have the 1080 option available.  Possibly you may need to reboot to set the changes.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project. 
If deleting preferences does not cure it,  try doing an Edit/Select All, Edit/Copy of the media in your project, and then Edit/Paste it into the timeline of a newly created empty project.  See if now you can export at 1080p.  
Also, you will not have the 1080p export option available unless your project is 720p or higher.   The project resolution can only be set when the project is first created, by inserting a 1080p clip as the very first video clip added to the project. You can thereafter remove the clip if you want, or add clips other than 1080p, and the original 1080p project resolution will not be changed.  
-- Rich