You made the screen too small and the font too big for it to move down from 65,100.
With CLS and SET CURSOR in the DO/LOOP it's always going to be in the same spot because it clears the screen and resets the cursor location to 65,100. If you put the CLS and the SET CURSOR outside of the DO/LOOP and rem off the screen size you'd see the text moving down from 65,100.
`Set Display Mode 200, 200, 16
Set Text Font "Arial"
Set Text Size 80
Cls
Set Cursor 65, 100
Do
key = scancode()
If key <> 0
Print key
wait key ` To show it working
Endif
Loop