How can I access my copy of Minecraft from a new Xbox account?
You cannot access the content if your old account isn't currently signed in. You can only access the content if the account that purchased the content is signed in.
I did something similar with some DLC my friend had purchased on the same xbox. I signed in on their account, then signed in my account (at the same time), and I had access to the DLC.
Therefore, since the original account is not signed in, the xbox is reading that you (current profile) do not own this content.
EDIT: Here is a similar answer covering the requirements of the console and user licenses.