Why does Slim have the respect of the men on the ranch in Of Mice and Men?

Slim has the respect of the other workers on the ranch because he is very skilled, wise, thoughtful, sympathetic, and, above all, fair. 

Slim is both large and tall. He is a "jerkline skinner" who is capable of driving as many as twenty mules with only a "single line to the leaders" (he has such skill handling the mules that he does not need as many reins as others do). He is so capable that...

Slim has the respect of the other workers on the ranch because he is very skilled, wise, thoughtful, sympathetic, and, above all, fair. 

Slim is both large and tall. He is a "jerkline skinner" who is capable of driving as many as twenty mules with only a "single line to the leaders" (he has such skill handling the mules that he does not need as many reins as others do). He is so capable that all the men admire him. He is impressive because of his size as well. Unlike many people, he listens conscientiously to others and actually hears more than is said; also, with his "God-like" eyes he sees beyond the surfaces of things. Further, he is placed above the other men because he

...moved with a majesty only achieved by royalty and master craftsmen. He was ...prince of the ranch...His authority was so great that his word was taken on any subject, be it politics or love.

In Chapter 4, Slim is the only one in whom George feels comfortable enough to confide; furthermore, Slim commiserates with George regarding his care for Lennie. Slim understands that George must be patient with Lennie and allow for his handicaps, as well as be vigilant in keeping Lennie out of trouble. In the final chapter, it is Slim who consoles George, telling him, "You hadda, George. I swear, you hadda," and he walks George back with him up the trail.

It is also Slim who is sympathetic to old Candy. After Carlson suggests putting the dog out of his misery, Slim studies the old dog "with his calm eyes." he tells Candy that he can have one of the puppies born to his dog. Then, he explains calmly to Candy that his old dog is "no good to himself," meaning that the dog is in pain from being crippled by arthritis. "I wisht somebody'd shoot me if I got old an' a cripple," he sympathetically explains to Candy as the old man looks helplessly at him because "Slim's opinions were law."

Slim is always fair with everyone; he does not flirt with Curley's wife, yet he is nice to her. He maintains a certain standing among the men at all times as he is judicious and kind. Because he treats everyone equally and is skillful, wise, and kind, Slim is well respected by all.