Why were spain's monarchs interested in the proposal columbus made to them?

Simple answer, gold, power, and riches. They liked to buy things and build castles but it cost a great deal of money and the treasury needed a bit more in it. There was also an interest in finding a water route to Asia for the goods that could be sold in Europe. Better trade meant getting more stuff, more stuff meant getting more money. Power was also involved because finding new routes, lands, and riches meant the crown was expanding its influence in the world. More land meant more power and the more power they had the richer they got. At one point Spain considered the Pacific Ocean their territory and didn't want any other ships sailing in the Pacific.