Chicken and egg. Not enough customers for them to bother changing anything, not enough merchants for most non-bitcoiners to want the option.
They'll come on board once bitcoin is bigger. Better to focus on companies more interested in getting in early.
No one is going to to buy Bitcoins just to spend them on Amazon. Not a chicken and an egg problem, simply a sign of relatively low adoption.
They will use bitcoin to spend on amazon because it will reduce overall costs verses using a credit card