When you purchase a DLC, you are purchasing a license to use on that platform it is tied to your platform, not your Bungie account. Bungie and Blizzard sent emails and Bungie referenced this subject in TWABS for 14 months plenty of notice was given. The game licenses were attached to Blizzard and they offered a 1-time chance to transfer over, this had to be done before 1st December 2020. Unfortunately, nothing can be done now.