Along the Official Announcements;
A direct upgrade option was initially planned for Zorin OS 12 to Zorin OS 15. Due to time constraints and the (at that time) bringing in of outside contractors to ensure the finalization of Zorin OS 15, ZorinGroup needed to set a project on the back burner and it was deemed that the release of Zorin OS 15 took higher priority.
Direct Upgrade option was set on hold for almost a full year.
Work on the project has resumed and in the following posts, The ZorinGroup acknowledged and confirmed the work has resumed at High priority: