> The use of the HJKL keys for moving the cursor in the vi editor and its descendants originated from the ADM-3A [...] The ←, ↓, ↑, → and Home labels printed on the H, J, K, L, and ~ ^ keys were a visual reference to the control characters Ctrl+H, Ctrl+J, Ctrl+K, Ctrl+L, and Ctrl+~ ^ that were required to move the cursor left, down, up, right, and to the top/left corner (or "Home" position) of the terminal, respectively [...] The Ctrl+H and Ctrl+J functions were the standard ASCII backspace and line feed respectively, but the interpretations of Ctrl+K, Ctrl+L, and Ctrl+~ ^ were new to the ADM-3A.

Seems almost a happy accident. I personally like the placement because the most common movement in a document is down and it's placed right under the most dextrous finger, the index finger, by a coincidence between ASCII encoding and QWERTY layouts. Up being the second most common movement was put under the second most dextrous finger. Nice.