why does Darry work so hard?

Since their parent's deaths, Darry is responsible for taking care of the family. He works hard because he because he wants to take good care of his brothers.

because their parents died and darry works to get the money to keep everyone alive and well

he is the head of the house hold he takes up all the responsibility for his brothers.

He wants to be responsible because that's his job, to be responsible and civilized. He doesn't want anything to happen to his brothers.