The search of the American Dream is a prominent theme found in John Steinbeck's novel Of Mice and Men. Unfortunately, for many of the men (especially Lennie and George), their dreams came crashing around them.
In regards to the novel, there are many things which keep "the poor" from realizing their dreams.
1. Lennie's diminished mental capability. Given that Lennie and George cannot keep a job, because Lennie keeps getting in trouble, the men are forced to move from place to place. They are simply unable to build up a "stake" in order to purchase their own land and be their own bosses.
2. Crooks' race. Crooks, after hearing about George and Lennie's plan, wants in on their dream. While he has money to give, the fact that he is black simply underlies his inability to make his own dreams come true. He cannot ever be rich given his race. He will, inevitability, be poor and unable to escape the ranch.
3. Curley's wife. Curley's wife is simply a woman. Her "wealth" only exists based upon the fact that she is married to Curley. In fact, she is not even given a name. Instead of being a person, she is a possession. She, literally, has no value or worth. Therefore, she will never be able to see her dreams come true.
I think Steinbeck would have said that there are too many poor, ignorant, unskilled people competing for a limited number of jobs, as they were in The Grapes of Wrath. Because of the working of the law of supply and demand, wages are forced down, and the lowest classes can only earn enough to keep themselves and their children alive. They are victims of "wage slavery."