Because the monopolist doesn't pay attention to the willingness to pay of inframarginal consumers:

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When a monopolist does not pay attention to the willingness to pay of inframarginal consumers, there can be a difference between what level of product quality is profitable for the monopolist and what level of product quality maximizes aggregate surplus. This is because the monopolist may choose a lower quality product that is profitable for them, but not necessarily the quality that would benefit consumers the most and maximize total welfare.

Because the Monopolist Doesn't Pay Attention to the Willingness to Pay