Why "thee" and "thou" etc. in 19th century writing?

In the 19th Century, authors were still using what is considered "King James" English -- that is, old English. The use of the pronouns thee and thou in the place of the word "you" was not uncommon, and it was equally not uncommon to see other language used that today we consider obsolete (ye, thy, forsooth, etc.).

Granted, plenty of American authors had strayed from the old English spellings and usages during this same era, but if an author felt the need to give a piece a more formal tone, the use of King James English was not unusual.