Because I checked the documentation regarding the use of '\dD', and there was no explanation for this special case.
Because it is not a special case of \dD specifically but a behavior of how all the \d meta-commands function. This behavior is documented under Patterns (here, the absence of specifying any pattern):
''""
Whenever the pattern parameter is omitted completely, the \d commands display all objects that are visible in the current schema search path — this is equivalent to using * as the pattern. (An object is said to be visible if its containing schema is in the search path and no object of the same kind and name appears earlier in the search path. This is equivalent to the statement that the object can be referenced by name without explicit schema qualification.)
"""
Your new domain is not visible since the plain type numeric in pg_catalog is implicitly first in the default search_path.
