If he had left for WWE a long time ago when WCW folded, it would be a different matter. They had Austin, The Rock, Angle, and were firing on all cylinders in terms of creativity. Today they absolutley suck in that department.
I saw him say in an interview, he spoke to Vince but was less than confident he would be (or his character) would be used the right way. Looking at a number of performers who went to WWE, he has a right to be skeptical. Vader, Booker, Steiner among others have been horribly used by the company in spite of their abilities as performers. The politics are a huge problem and most people who have been there will tell you, they have a hard time being allowed to voice their opinions about how to tweak their characte. Why should he leave TNA, where he is allowed input to go to a totally dictatorial type of environment ? He has made a lot of money, he has a decent schedule, I'd stay with TNA, he doesn't need WWE. It might be helpful to go there and do a 'Best of Sting' DVD, since Vince does own the footage of him in WCW, other than that I wouldn't go there.