For a few reasons:
- event.preventDefault() for the Enter key (to stop it from submitting
the form when you select an item) is only effective if it's a keydown
event.
- Using keydown for up/down navigation means you can hold them down to
rapidly scroll through a lot of items.
- Keydown events make the UX feel more responsive, and is consistent
with how most apps/operating systems handle key presses.
Jeff Sagal