They are getting rid of it because the USAF managed to get its way and screw the USA over again.
The USAF NEVER wanted the C-27J*... it was bought into as a "US Army only" replacement (Future Cargo Aircraft, 2004) for the ~45 C-23A/B/C (Shorts Sherpa/Sherpa 300) which the Army had gotten (initially) from the USAF because the USAF didn't want to keep
them anymore.
Panel cuts Air Force funding for JCA - Air Force News | News from Afghanistan & Iraq - Air Force Times
One military observer said the Air Force’s interest in the C-27, which could be mistaken for a pint-sized C-130, was more about protecting the service’s fixed-wing turf and less about the service’s need.“The requirement originated in the Army rather than a joint need,” said Loren Thompson, a defense analyst at the Lexington Institute in Arlington, Va. “I never felt the Air Force’s interest in the C-27 was more than a desire to protect its airlift role.”
The USAF saw Congress express a likelihood of approving 33-75 "Future Cargo Aircraft" for the Army and began throwing a "turf war" fit over the Army moving into fixed-wing tactical transports (the C-23 counted as a "liaison aircraft", which the Army is allowed to operate). In August 2005, General John Jumper, then Air Force chief of staff, summed up his feelings regarding the Army's Future Cargo Aircraft during a roundtable discussion with reporters by stating, "My thought on that is you don't need to go out and buy yourself an Air Force - we've got one."
In September 2005, the USAF declared they were looking for a 'Light Cargo Aircraft" (USAF Generals had been "discussing" whether they had a need for such an aircraft for all of 2-3 months, only after the Army sought industry input on a replacment for the aging C-23 Sherpa fleet), and that the same aircraft should be bought by both services (in December 2005 DOD directed that both programs be merged, with the Army as "lead").
The USAF then convinced Congress to let it have 70 of the 145 (up to 200) aircraft buy (Joint Cargo Aircraft program, June 2006) AND to transfer "lead" on the reduced-to-78 aircraft program (April 2009) from the Army to the USAF.
Then the USAF got Congress to eliminate the Army part of the buy
without adding them to the USAF part (December 2009)... and finally got the program killed altogether (2012)...
after ~1/3 of the buy had been delivered, and the aircraft had made its first Afghanistan deployment!
*
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA539920
This paper discusses repeated analysis by the USAF claiming that airlifters smaller than the C-130 were "less efficient than the same number of C-130s'.
While the Air Force has done some recent studies to find specific scenarios where a C-27J might be optimal, most of the work done has shown that the C-130J is more cost effective over a variety of mission areas.
Examination shows that the "specific scenarios" that favored the C-27J were the missions the Army wanted it for, while the "variety of mission areas" included many USAF-specific missions.
It also contains a good "capsule history" of the program on pages 27+.