To answer this question, take a look at Chapter One when we meet George and Lennie on their way to the new ranch. From their conversation, we learn that they were forced to leave their previous ranch because Lennie got into trouble. Specifically, Lennie wanted to stroke a girl's dress because it felt soft. However, she believed that he was trying to rape or hurt her, which resulted in George and Lennie having to flee the ranch and find work elsewhere.
Another reason for going to the ranch is financial. George and Lennie have a dream of owning their own ranch and must save up enough money if they are to make this dream a reality. The new ranch, therefore, will provide them a living while they get the money together to buy a place of their own.