Yeah, there are bits on the Wikipedia page about possible reasons, but he and Kamelot are being (understandably) discreet.

I do wish we knew more, if only so we could know whether to expect him him to continue in the music business. As for Kamelot, I think I'd actually like it if they pulled a Nightwish and found someone with a different style. I don't think Khan's voice defined Kamelot the way Tarja's did Nightwish, but personally I think I'd like it better if they found a new person who was strong in their own way rather than trying (failing) to imitate Khan.