Both Lennie and Crooks attract attention because of their physical appearance (Crooks being black and disabled, Lennie being large). Their appearance gets them into trouble - Lennie's size with Curley; Crooks' colour with everyone except Lennie. Both men are subject to the prejudices of the time: racism and a misunderstanding of mental challenges. They are both viewed with suspicion and resentment and are subject to humiliation. However hard either man tried to overcome their challenges, they would not be able to escape their position.
Both Lennie and Crooks live isolated existences on the fringes of society -- Lenny because of his mental disabilities and Crooks because of his race. They also both represent a section of society that seems to have little chance of upward mobility. They work on the farms and in the factories but don't and probably never will own any part of them.
Despite their low station in life, both Lennie and Crooks seem to carry a kind of dignity with them. This is a trait Steinbeck was masterful at portraying -- dignity in the face of the harshness of life.