Because over time, and over the islands the beaks and other matters of the finches changed (size, shape, etc.) These changes were later discovered by Darwin to have these adaptations to better access food sources on whichever island they were located. For example, One type of finch had a short strong beak meant for cracking seeds, while another had a long narrow beak for gathering food from the inside of flowers, and cacti, therefore making each species more adapted to its specific environment and more able to gather food then they would on another island with a different main food source. (sorry its long)