What is the resolution set for the project? Click on the Settings button to the right and above the timeline. Look at the little box at the top of the dialogue box that appears to find out the resolution set for the project. If the project resolution is a lower number then your export options will be limited. You would need to create a new project and put a known 1080 video (not photo) clip into it as the first clip added. That would set the project at 1080. Then close the project, reopen, and delete the clip.
If the above is not your issue, then try deleting 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 get a 1080 share option.
-- Rich