Thinking historically, the big Air Force doesn't care for the tactical fight, but it doesn't want to let the Army have the tactical airframes. As can be seen by the F-35 can fill the role of the A-10 in the fight. Although I will point out that the A-10 wasn't truly built to just hang around, it was built to be tactical support for an armor-on-armor fight. The big Air Force likes flying fighters and bombers, historically speaking.
There are a myriad of reasons why it would be a bad idea for the conventional Army to own CAS aircraft, and to train their own JTACs. Yes, big blue likes flying fighters and bombers. Luckily, the red headed step children of the USAF continue to hang around and enjoy their role as outsiders stuck between two services.
