In the 1930s, people with mental disabilities were not treated with a great deal of sympathy or respect in society. In general, they were removed from their families at a young age and placed in large institutions, usually run by the state, in which they were not educated. Instead, they were often given menial work to do and were given limited freedoms, and they did not often see their families. These institutions were often large in size, and the staff usually did not get to know the inmates personally. The staff did not regard people with disabilities as worthy of empathy and help, and they did not train them to gain skills that could help them go on to lead independent lives. Often, inmates were subjected to mental and physical cruelty, but they had few advocates in society to stand up for their rights to respect and dignity. Therefore, in Of Mice and Men, George believes that Lennie will live a far better life outside of an institution rather than in an institution. Fortunately, the situation began to change in the 1960s, when there was a movement to provide more rights and independence to people with disabilities.

Mentally disabled people in the 1930's did not have the same treatment which they are afforded today. Mentally disabled people during this era tended to be placed into mental wards--at an alarming number. Doctors during this time did not have the understanding of mental disabilities that they do today. Many hospitals were overcrowded given doctors would tend to commit the mentally disabled person (given their inability to properly treat them).

An a side note, the lobotomy became widely popular during the 1930's (in attempts to study the brain in a different way). The original study was performed on two monkeys (Becky and Lucy).

One could easily assume that George worried about Lennie being placed into an overcrowded mental facility and this is why he chose to keep Lennie with him.