Don't really have to be privy to reasons. Yes, he was sentenced to death, but keep in mind there has been no active duty service member executed since 1961...56 years ago.
In military cases, there are several mandatory appeal stages, and a military death sentence requires final approval by the president, as commander in chief.
A civilian attorney who has assisted Hasan in his legal matters says that the appeals process could go on for decades. He received the death sentence in 2013...that's only four years ago. The chances of Hasan actually being executed are about as close to zero as you can get.
Keep in mind, also, that Hasan is paralyzed from the chest down...the result of being shot by military personnel. The chances are actually better that he'll die from some medical issue rather than being executed.