Why did people in the past try so hard to be like everyone else?
The American culture of the 1950's has been labeled as an era of conformity by historians. Conformity is the word for the idea that you have proposed in your question. Since the Fifties was an era of conformity, it should come as no surprise that psychologists wanted to study the phenomenon. In 1955, Morton Deutch and Harold Gerard published the results of a study on why people conform. They identified two classifications for understanding why people follow the group. First is what they called normative conformity. What this means is that people have a desire to fit into the groups and are naturally fearful of being rejected. In this situation, the individual may, in fact, disagree with the thoughts of the group but will not demonstrate this publicly.
The other reason that people follow the lead of the group is what they referred to as informational conformity. In this scenario, a person may be ignorant or uninformed and looks to the group to find the norm. In other words, the individual does not know any better so they are likely to follow the lead of others. The person that follows the group in this case is more likely to adopt and internalize the ideas of the group.