The Omani ruler in question here is Sa'id ibn Sultan I/II. Note that Sa'id ibn Sultan I and II are the same person. His first reign was from 1804-1806 as co-ruler with his brother Salim ibn Sultan. His second reign was from 1806 until his death in 1856 as the sole Sultan of Muscat and Oman.
Under Sa'id, The capital Muscat's importance as a harbour seriously declined, mostly due to the relationship the father of Sa'id, Sultan ibn Ahmad, had established with Great Britain. During the War of the Second Coalition, Napoleon Bonaparte planned to disrupt the British access to Indian trade through his campaign in Egypt and several British-French naval encounters occured soon after in the Western Indian Ocean. The British realized that Muscat, with it's central location in the Arabian Sea, would be an ideal staging point for a French attack on India. The British sent diplomats to Muscat in 1798 and Sultan ibn Ahmad pledged his friendship to the British and promised to keep the French out.
One reason stated for the decline of Muscat was that Sa'id's attempts to profit from this established relationship with Great Britain kept backfiring. Sa'id tried to get British aid against Oman's Arab rivals, but Britain ended up recognizing these competitors instead. The alliance with Britain also resulted on multiple occasions in French attacks on Omani trade. The main reason for the decline of the city, however, was the increasing British pressure to abolish the slave-trade in the mid-19th century. Muscat's population fell from 55,000 to 8,000 throughout the mid- to late-19th century as families migrated to Zanzibar. Sa'id ended up abandoning Muscat and leaving a regent in power in 1840, moving the capital to Stone Town (Zanzibar) where he put his slaves to work on clove plantations.
Upon his death in 1856, his sons split the power with Mayid ibn Said al Said becoming Sultan of Zanzibar, while his brother Thuwaini ibn Said al Said tried to restore the legitimacy of their dynasty as Sultan of Muscat and Oman.
ALLEN, C.H., 'Oman: A Separate Place', The Wilson Quarterly, 11 (1987), 48-63.
METZ, H.C., Persian Gulf states: country studies, Washington D.C., 1994. [Specifically chapter 6: "Oman"]
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