Why weren't the people on easter island able to grow their own food to survive ?

Because they caused soil erosion, which made the soil inappropriate to plantations.

Explanation:

Large areas were cleared for agricultural land use during the Easter Island population boom, which reached 15,000 inhabitants. By ambition, competition or even lack of knowledge, the natives did not know how to manage their resources in a sustainable way, cleared the entire forest and began to have problems with agriculture.

With the loss of the forest, the sources of water, important for irrigating the crops, have dried up. In addition, the rain and the sun eroded all the soil and this made it impossible for the inhabitants of Easter Island to grow their own food.