The primary reason that Columbus was rejected by Portugal was financial. Columbus offered to sail directly west to reach the Indies and Cathay, otherwise known as China. King John II rejected the proposal because he believed Columbus had miscalculated the distance of the trip. Portugal, now with favorable trading routes around Cape Horn, favored sailing below Africa. King John II employed astronomers that concurred with his belief that sailing directly west would be too far. They believed that the crew would run out of food and resources on the trip and that it would be too costly.
Plans such as this trip had been offered to Portugal in the past. They too had been declined. When Dias reached the Cape of Good Hope, Portugal obtained a direct trade relationship. Although Columbus reached a deal with Spain for the voyage, Portugal was technically right about the distance. Moreover, Portugal had already established a successful trading route along Africa and did not need to take the risk.