The original reason was that as the price increases, the product will reach a point where it begins to attract more and more substitutes.
The situation is linked to a United States Supreme Court case and a subsequent response in economic literature.
The Court agreed with du Pont that when evaluated at the monopolistic price observed in the early 1950s, there were many substitutes for cellophane and, therefore, du Pont had only a small share of the market for wrapping materials (i.e., it possessed little or no market power).
As Richard Posner wrote, "Reasonable interchangeability at the current price but not at a competitive price level, far from demonstrating the absence of monopoly power, might well be a symptom of that power; this elementary point was completely overlooked by the court"[3] The problem continues to bedevil efforts by antitrust agencies to define markets.
So we can't just use the price a company already charges as the base level, or we will conclude that even monopolists lack market power.