Why did George not tell them about Lennie's handicap? They would have hired him anyway because he is so strong.
Be careful not to make assumptions based upon values held in the present time. For, in the 1930s there was not the tolerance for disabilities that there is in the 21st century. Also, during the Depression there were so many physically and mentally "normal" men that there was no need for any employer to make allowances for anyone else.
Added to these conditions, George does not want to draw any attention to himself and Lennie. He knows that the other men might make fun of Lennie, and he fears that Lennie may say the wrong thing. Nevertheless, when the swamper asks suspiciously, "Why don't you let him talk?" George does admit to Lennie's not being too "bright," saying that Lennie got kicked in the head by a horse when he was a boy and "Just ain't bright. [But] He can do anything you tell him." Then, when the swamper leaves, George scolds Lennie:
So you wasn't gonna say a word. You was gonna leave your big flapper shut and leave me do the talkin'. Damn near lost us the job.
George adds, "Now he's got his eye on us. Now we got to be careful and not make no slips." George traveling with another man looks odd in the first place and his having a friend who is not the same arouses suspicions that he is exploiting the other man in some way. In desperate times people are more suspicious of one another and wary of anyone who is a little "odd."