Almost every religious office can be filled again. This one cannot, and the reason is a matter of constitutional design rather than of decline or disuse.
The authority to name a Hand of the Cause belonged to the Guardian of the Baha'i Faith alone. Shoghi Effendi held that position and made his appointments in stages through the 1950s, the last and largest in October 1957. He died in London weeks later, without a will and without naming a successor, and the conditions the Baha'i writings set for the Guardianship could not be met by anyone else. The power to appoint therefore ceased to exist at that moment.
The consequence is exact. Those already appointed continued to serve, and served for decades. No new Hand of the Cause has been named since 1957, and on the Baha'i understanding of its own texts none can be.
The office itself carried no jurisdiction. A Hand of the Cause held no authority over any Spiritual Assembly, country or region; the charge was global, exercised by teaching, by defending the Faith against opposition and internal dissent, and by counselling the elected institutions. It was a rank of individual distinction rather than a post in a chain of command, which is why its closure did not leave an administrative vacancy so much as a functional one.