Has he? He know has something he's probably wanted for a long time: attention. He's writing books, going on news broadcasts... after his plan to join the CIA washed out, this seems to be Plan B. Assuming he dodges jail, this is probably putting him on the path to a political career.
IIRC, he confessed to the crime the first time during a polygraph test he took as part of his application to join the CIA. They turned him in, there was an investigation, he had some admin actions taken against him but no jail time.
Then, during an interview, he admitted it again, possibly because he believed double jeopardy was attached.
At any rate, who can say why anyone does anything? There are several documented cases where people
confess to "war crimes" that they never even committed.
As far as why he did the crime itself, it's amazing what people will say or do when they think they're operating in the absence of consequences. There seems to be a lot of that in the SOF community right now.