Why did both the minoans and mycenaeans both come to depend on trade

There was no unified state that could grow all parts of life equally so they had to depend on trade in order to get goods that they needed from each other since they couldn't develop all parts of production or manufacture. Ones would for example have a developed pottery industry while the others would have mastered making Olive oil so they would trade in order to get what they needed because they couldn't make it on their own.

The Minoans and the Myceneans both depended on trade due to the scarcity of raw materials and other resources in their own territories.

The Minoans were an island civilization, and all island civilizations need trade to be successful due to the scarcity of materials on the island. 

The Myceneans had the same difficulty on mainland Greece. It imported raw materials from all over the Mediterranean, and then used its developed infrastructure and society to manufacture and export finished goods.