If NASA had bought into the Evolved Expendable Launch Vehicle (EELV) program, the Air Force was hoping that the purchases would drive down the costs per rocket to save them some money. Depending on the configuration needed to put payloads into orbit, EELV launch cost can range from $100 to $250 million. NASA would have had to spend lots of time and money "man-rating" an EELV configuration as safe for use in manned space flight.
The EELV program currently uses two launch vehicle families, the Boeing Delta IV and the Lockheed Atlas V. Either family is currently capable of lifting anywhere from 9,300 to 28,000 pounds to geosynchronous orbit. NASA's heavy launcher needs to be capable of lifting up to 220,500 pounds to low earth orbit. NASA's engineers already have a better grasp of the ins and outs of both the capabilities and costs of the Shuttle's solid rockets and liquid-fueled engines, so building a derivative vehicle using those components would be faster than having to pick between the Delta and Atlas and learn all of its capabilities from scratch.
It's not the first time the Air Force has failed to find a cost-sharing partner. Originally, the Air Force and the aerospace industry assumed there would be a booming market in launching communications satellites, so private sales of the EELV would drive down rocket costs.
In the '70s, NASA and the Air Force were forced to work together on the Space Shuttle program to push costs down. Expendable rockets were expensive and the Air Force thought it would be able to save money by using the reusable Shuttle to launch satellites. However, the Air Force added a number of requirements so it would be able to launch the space shuttle from Vandenberg Air Force Base, used to launch large spy satellites into polar orbit. But those requirements ended up driving up the cost of the shuttle further. Production for other rockets was terminated in order to push up the shuttle launch rate and lower the cost per launch.
The loss of Challenger in January 1986 forced both sides to re-evaluate using the Space Shuttle system. Satellite payloads would once again be sent up by expendable rockets instead of in the Shuttle's cargo bay. The Air Force's billion dollar investment in the Vandenberg shuttle launch facility was written off, the facility never used for an "All Blue" Air Force launch. Ultimately, the Air Force embarked on the EELV program to drive down the cost of larger expendable rockets. Doug Mohney
The Pentagon had hoped NASA would buy into its heavy rocket program, to go back to the Moon and on to Mars. But the space agency has determined that it would be too expensive. Instead, it will use rockets derived from existing Space Shuttle systems for both manned, and heavy launcher unmanned launches. Numerous studies came to the conclusion that Shuttle derived systems utilizing the program's solid rocket boosters and liquid-fueled engines could fly sooner, safer, and for less money.