In the latest edition of Live Audio Wrestling, which you can check out in full at liveaudiowrestling.com, Samoa Joe had the following to say about why he left TNA Wrestling:

“I think the decision was made in the final few weeks. I’ve often been called by people loyal to a fault, and I didn’t feel the mission was accomplished in TNA yet. I’d done my best throughout my career to help try to build the company and bring it into prominence. It’s had its ups and down and pitfalls here and there, and I don’t feel that job was finished yet. Towards the end there, there were some apparent things and incidents that kinda showed me that maybe it was time for me to move on and maybe it was better for me, both personally and professionally, to explore other options and explore other things. I’ve gotta admit, I’m not unhappy with my decision. I’m unhappy that I didn’t get the opportunity to kinda finish out what I wanted to start in TNA, but at the same time, so many great things have popped up since that I’m very, very satisfied with the decision that I made.”

http://fightnetwork.com/news/law/

Basically "I no longer had any faith in TNA and I jumped at the right time. I don't regret trying, though."

Leaving TNA was a smart move by Joe.

I still say his biggest moment was when he won the title in 2008

Always annoyed me TNA never used him right ever since he lost the World Title <_<