Final answer:
Portugal rejected Columbus's plan to sail west to Asia because they had success with their existing trade routes around Africa, established by Bartolomeu Dias and Vasco de Gama, and did not see the need to invest in Columbus's uncertain alternative.
Step-by-step explanation:
The Portuguese crown rejected Columbus's plan to sail west to Asia because they had already established a route to Asia around the bottom of Africa. Portuguese mariner Bartolomeu Dias had discovered a route around southern Africa, and later Vasco de Gama sailed it to reach Asia in 1488 and 1497, respectively. At the time, Portugal was more focused on consolidating its power along the established African trade routes and was not interested in speculative ventures in unproven directions.
Furthermore, Portuguese dominance on the African coast and their control over the existing lucrative trade routes made Columbus's proposition less appealing. At the time, Portugal was heavily involved in the spice trade and slave trade, which yielded significant profits. The Portuguese had less incentive to invest in Columbus's uncertain plan when they could capitalize on the known routes and markets.
Meanwhile, after completing the Reconquista, Spain, under the rule of Isabella and Ferdinand, chose to fund Columbus's expedition in 1492, motivated by the competition with Portugal and the desire to find a new all-water route to Asia, as well as spread Christianity.