Why doesn't keypress handle the delete key and the backspace key

The keypress event is designed for handling a character typed by the user rather than detecting keyboard activity and the delete and backspace keys do not generate characters. Some browsers blur this line somewhat but the general principle is that the keyup and keydown events are there to detect any key being pressed and telling you about which key it is while keypress is for detecting an actual character being typed.

The short answer is that the onkeypress event is not fired for all key types in all browsers. Check the manual for your browser.

Why?

Probably not a comprehensive answer, but consider Shift, when it goes down and when it comes up relative to other keys is significant. And different keyboard hardware has different key rollover characteristics which you might want to know about in detail.