Why doesn Josephus go into greater detail about Jesus “marvelous deeds,” instead of just tantalizing us?
Because Josephus is following the source text before him, which does not go into detail. Luke only writes Jesus was “mighty in deed.” Perhaps Josephus did not know anything about the deeds of Jesus, but we cannot tell if that is the case, as Josephus often closely follows his sources, allowing only stylistic variation, evidently to prevent charges of distorting the record. His desire to follow his source thus does not preclude the notion that he has reason to trust the source as authentic, as valid as any of the other historians whose work he employed in the Antiquities.
