The port of Surat declined by the end of the 18th century because
1. By that time, the British had become the predominant economic power in India. They conducted most of their trade from Calcutta, Bombay and Madras. Surat could not compete with these new ports.
2. Simultaneously, the Mughal empire declined. Surat had been the main port of the Mughal empire. With the decline of the Mughal empire, the importance of Surat also declined.