TLDR; Ctrl+J inserts a newline in Readline because J is the 10th letter of the alphabet and the tradition of all the dead generations weighs like a nightmare on the brain of the living.
Using Ctrl+J to insert a new line has a history that spans hardware engineering, OS design, and early text editors. Its lineage can be traced through three major layers: ASCII and Teletype hardware, Unix TTY drivers, and Emacs/GNU Readline.