A monopolist is able to earn supernormal profit in the long run primarily because of

The monopolist maintains supernormal profit in the long run because high barriers to entry — such as patents, licences, or control of key resources — prevent new firms from entering and competing away the profit. Without barriers, new entrants would erode the monopolist's profit, as happens in perfect competition. Price discrimination (D) is a strategy but not the primary reason for sustained supernormal profit.