Columbus had already been turned down by the kings of Portugal, France and England in his appeal for funding for his voyage, so of course the Spanish Monarchy was also hesitant to get on board with his plan.
Furthermore, Queen Isabella was a genuine religious zealot, and the fact that Columbus claimed that his 1492 voyage would prove that the earth is round, was a point of consternation for the Catholic Monarchy. It took Columbus five years, and the backing of a religious friend of the court, to convince them. The dire need to find a new route to Asia, coupled with the desire to spread the Catholic religion to new territories, eventually won over the king and queen.
