The Government Accountability Office (GAO) issued a fairly scathing report about the Air Force's plans to retire the A-10, accusing the USAF of incomplete research about the cost savings and tactical implications of retiring the aircraft.
The overall tone of the GAO's report about the retirement of the A-10 is best summed up by this quote from the report's conclusion (emphasis mine):
Overall, the Air Force did not meet all best practices in estimating cost savings from A-10 divestment, which affected its ability to determine comparable alternatives. In its fiscal year 2015 divestment proposal, we found the Air Force’s cost estimates partially met best practices for being comprehensive, minimally met best practices for being well-documented and accurate, and did not meet best practices for being credible. Because the Air Force’s cost estimate did not meet best practices in these areas, the 2015 proposal potentially overstated or understated the actual savings from A-10 divestment. Additionally, Air Force officials stated they used similar practices to estimate cost savings when developing budget requests for fiscal years 2016 and 2017, thereby continuing to potentially overstate or understate the actual savings from A-10 divestment."
In addition to the poorly-done cost savings estimates, the report indicates the mission vacuum that the A-10 could leave (emphasis mine):
[W]e found that the Air Force has not comprehensively assessed potential mission capability gaps caused by A-10 divestment or the effects of divestment on its ability to support Joint Terminal Attack Controller training. As we describe in our report, though the Air Force and DOD are taking steps to mitigate potential gaps, they have not established clear requirements for the missions that the A-10 performs, including CAS, FAC(A), and CSARSandy, and in the absence of these requirements, have not fully identified the capability gaps and risks that could result from A-10 divestment.
The retirement of the USAF's 283 A-10s by 2021 would also require an increased reliance on current 4th-generation aircraft to undertake the A-10s missions and with the Air Force facing very real financial problems in the coming five years, that is looking unlikely. The A-10 is very effective at its ground support missions, and that effectiveness combined with its lower operational costs is going to be a hard asset to replace.
In addition to the financial reasons, the GAO report also points out that the A-10 community is considered the Air Force's close air support experts, due to their focus on it during training.
You can read the whole report as a .PDF on the GAO's website