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Why did Columbus believe that Cuba was part of mainland Asia?

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Map showing the transatlantic and global routes of major early voyages of discovery and exploration by Spain, Portugal, England, and France during the 15th and 16th centuries. The map traces the journeys of explorers such as Columbus, Cabot, Magellan, da Gama, and others, with labeled paths crossing the Atlantic and encircling Africa and South America, as well as lines of demarcation dividing Spanish and Portuguese exploration zones. — European exploration Map depicting the European exploration of the New World in the 15th and 16th centuries, including the voyages made by Christopher Columbus, John Cabot, Alonso de Ojeda and Amerigo Vespucci, Pedro Álvares Cabral, Ferdinand Magellan and Juan Sebastián del Cano, Giovanni da Verrazzano, Jacques Cartier, Sir Francis Drake, and others. The lines of demarcation represent an early division between the territory of Spain (to the west) and Portugal (to the east).

Christopher Columbus believed that Cuba was part of the mainland of Asia—specifically Cathay—due to a series of flawed geographic calculations and a reliance on outdated or misinterpreted sources.123

His conviction was rooted in several key errors:

Underestimated Distance: Columbus used an incorrect value for the length of a degree of longitude, which significantly shortened his estimate of the distance across the Atlantic Ocean.145
Geographic Misconceptions: He accepted Ptolemy’s exaggerated estimates regarding the eastward extent of the Asian continent.15
Influence of Marco Polo: By incorporating the lands described by Marco Polo into his own calculations, Columbus further reduced the perceived distance between Europe and Asia.15
Confirmation Bias: Having initially set out to find Japan (Cipango), Columbus convinced himself by November 1, 1492, that Cuba was the Asian mainland, even though he lacked evidence of the great cities described in his sources.3

Consequently, he maintained this belief throughout his first voyage, despite the absence of the expected wealth or imperial courts.53