As the english language has changed and warped to meet the needs of its speakers, the meaning and emphasis of certain words changes. When people begin to use a new set of words, they become more commonplace and loose power as a result. The Ngram above shows the relationship between a few words that would be considered formal to many people and some words that represent the informal version of each. During the 20th century the relative abundance of formal words switched places with that of the informal counterparts and since, informal words have been used much more regularly. That being said, our society is pushing towards less formal methods of communication because of the abundance of casual words that are found in literature.
In order to look at why this switch occurred, it is a good idea to think about the type of people that were contributing to the literature of the day. Before the 20th century, books and records were written by educated individuals or professionals more commonly than the average person. As a result, more formal works were produced and terms that our society would consider slang had not become widely popular. During the 20s when the country reached its financial high before the great depression, slang terms and those that would be considered more informal emerged as more prominent contributors to literature. As the 20th century progressed, the frequency of each showed how culture was influencing the way people communicated on paper, offering a more thorough glimpse into how they spoke in person.
Modern day society greatly favors informal methods of communication over those that would be considered old, dated, or strict. Terms like “all right” are much more common than “acceptable”, and while this is not a definitive measure of the formality of society, it shows a push towards informal communication and ultimately a reduction in the use of formal terms and phrases.
The reason for this shift can likely be attributed to the internet and the interconnectivity it provides. Communication has reached levels that are unrivaled and as a result, we are leaning more towards slang in written communication. Whereas at one point written language was exclusively formal, new forms of writing that exhibit casual language are beginning to emerge.