I was wondering why every message, letter, etc. in Ender's game has no lowercase letters. Is it just because it was written in the 80's or is there a better reason?
It has been the habit to send naval messages in all caps. It is just 2013 that US has began the process to stop this.
At press time it was estimated it be 2015 before the change could be fully tested.
It's just a formatting thing. A way for the reader to distinguish them from the rest of the text.
In the original 1985 edition they were also in a larger typeface than the rest of the text, this was changed in the 1991 revised edition to be just caps. See How does the 1991 edition of Ender's Game differ from the 1985 edition?.