The title, Of Mice and Men, comes from a Robert Burns poem "To a Mouse." One of the lines is "The best laid schemes o' mice an' men / gang aft agely." (The best laid plans of mice and men often go awry).
George and Lennie's plans (dreams) of the farm often go awry because Lennie's lack of social skills often leads to bad situations; sometimes someone gets hurt (or killed). Of course, this is unintentional but it is still a constant reality they must deal with. The larger reason their dreams are never realized is because they are itinerant workers. It is difficult for them to stay on a job for a substantial amount of time because of the nature of the job, but also because Lennie always gets into trouble.
Also, their dream has become so idealized that, even if they were ever to get a farm, it would never live up to their expectations.
One of Steinbeck's comments on dreams is that they are idealized or romanticized. In addition, the reality for Lennie and George is that their social position (being laborers with little education) does not give them much opportunity to pursue those dreams. Lennie's social awkwardness aside, migrant ranchers had a harsh life and upward mobility was not a guarantee no matter how hard one worked.
Crooks knows firsthand the social and economic obstacles in pursuing such idealized dreams. In Chapter Four, he tells Lennie what he thinks is the brutal reality or ranch life.
"I seen hunderds of men come by on the road an' on the ranches, with their bindles on their back an' that same damn thing in their heads. Hunderds of them. They come, an' they quit an' go on; an' every damn one of 'em's got a little piece of land in his head. An' never a God damn one of 'em ever gets it. Just like heaven.
The best laid plans go awry. This isn't because Lennie, George, or Crooks are not worthy or determined enough to achieve their dreams. There are external social forces (and internal if you count Lennie's mental handicap) that make it difficult if not impossible.