In addition to the answers provided to you previously, these are other reasons why the Caribbean was a good place to anchor a good part of their presence.
As far as the Antilles, their strategic position was prime for trade, natural resources, the creation of forts, defense, and war strategies. Spain, under the ruling of the Kings of Castille and Aragon (Fernando and Isabel) was responsible for affronting the money to Cristopher Columbus in the expedition that came to be known as "the discovery of America".
The resources that were found in the Caribbean were nowhere compared to those found by Hernan Cortez, but the availabilty of three different bodies of water (Caribbean Sea, Pacific Ocean, and Atlantic Ocean) surrounding these islands definitely proved beneficial for the establishment of new trade routes.
The same thing happened with the English as they began to establish their own trade routes once the colonies started to build a trade economy.