Because barring any further foreign orders in the next six months, the C-17 line is due to shut down permanently in mid 2015.
After that, if the USAF wants to buy more transports with larger capacity than the C-130J, it's either the A400M or a decade long new development program.
you are forgetting that a line can be reopened hence the C-5B. Or who is to say there is not already a replacement cargo bird in development in the works. We have enough C-17’s to last a long time. and more than enough C-130’s with upgrades and new C-130J models being bought.