Debuting their new look/design, the new $100 bill went into circulation on 10/8/13; the $5 bill on 3/13/08; the $10 bill on 3/2/06; the $50 bill on 9/28/04 and the $20 bill on 10/9/03. So...

1. why do you think they redesigned and issued the bills in this order--why the $100, followed by the $5, then $10, then $50 and then $20?
2. why do you suspect the $1 bill was never re-designed?