Why is there a fixed cost for travel to Bhutan? And why is it relatively more expensive than neighboring destinations?

The fixed cost that you pay is not just a fee for entry to Bhutan. This cost is a package tour cost that covers all your hotel, meals, transfers to and from the airport or border, transport for sightseeing, one knowledgeable local guide, all inland permits and fees. And if you are trekking this covers all expenses for the treks such as horses/yaks, guide, all tenting/camping equipment, meals and transport. The only way to visit Bhutan is by purchasing a tour with a reliable and licensed travel agency.