Most of the characters in Of Mice and Men have some kind of disadvantage, something that holds them back from leading fulfilling lives. Lennie has the mental capacity of a child and always gets into trouble because of his disabilities. He never means any harm to anyone; it's just the way he is. But in 1930s America, his condition is poorly understood, and so people think they have a license to treat him badly.
Crooks has the lowliest status of all the workers at the ranch. That's purely and solely because he's black. He finds himself trapped on account of society's racism. He hates the way he's treated at the ranch, forced to live separately from the other men. But he has no choice. There aren't many other opportunities for someone of his race and status in the world outside. So he has little option but to remain where he is.
As Lennie is disadvantaged due to a mental disability, so Candy is similarly held back due to a physical one. With only one hand, and getting on in years on top of that, Candy knows that it's just a matter of time before he's put out to pasture, so to speak. He's spent his whole life working for other people, without anything to show for it. Society regards someone of his age and disability as being virtually useless, and so it's not surprising that Candy jumps at the chance to be a partner in George and Lennie's ambitious dream of owning a ranch.