Formality is one main reason. For a long time, students were called by their last name, and people you didn't know would be offended if you used their first name to address them. When and why the trend towards informality with names happened I can't tell you and might be a question for /r/AskAnthropology

But there are many other reasons, and I think it has as much to do with being a literary figure as it is to do with the time period you choose. There are still plenty of authors today who use initials.

Simon Trewin, literary agent with Peters, Fraser & Dunlop, says the trend for initialisation is a throwback to the 1930s, 40s and 50s.

It was a time when writers and poets occupied a more sacred, hallowed status in society.

"It's all about formality. There was a time when giving out your first name was informal and unsuitable for the profession,"

http://news.bbc.co.uk/2/hi/special_report/1999/02/99/e-cyclopedia/486501.stm

Why does George Martin go by George R R Martin?

"It's a common name," he explains. "There was ‘the fifth Beatle,' a defensive end [for the New York Giants] and a million others. You want a distinctive name as a writer."

http://www.northwestern.edu/magazine/winter2009/alumninews/close-ups/georgerrmartin.html

Similarly:

• Robert -> R. L. Stine
• John -> J. M. Coetzee

Some of it is to hide your gender if there is a potential for bias (like J K Rowling's publishers feared)

• Mary Ann Evans -> George Eliot
• Phyllis Dorothy James -> P D James

and recently going in the other direction:

• Tom E Huff -> T E Huff / Jennifer Wilde

And some of it is just for anonymity. Also, some people simply don't like their first names and prefer to go by initials, or by their middle names. C S Lewis hated the name Clive, and went by Jacksie / Jack. But taking the pen name "Jacksie Lewis" would have put him at the opposite end of formality from what he chose.