The novel is titled 'Of Mice and Men' possibly to suggest the smallness and insignificance of the characters' lives and the constant struggle against circumstances beyond their control. It could also be a metaphor for the fleeting nature of their dreams and the harsh realities they face.
Well, it could be because the title reflects the fragility and vulnerability of the characters' hopes and plans, just like mice and men in a harsh world. It also might draw from a literary or cultural reference that adds depth to the story's meaning.