I think it might be to protect his legacy as the biggest draw even to never wrestle for WWE. If not he may been done with wrestling all together.
I think it's a combination of the schedule, money, creative control and Sting somehow thinking that something just does not 'feel right'. He has repeatedly stated his respect for Vinnie Mac and the he has been shown much respect likewise in the discussions they have had in the past. Moreover, he has good friends in the WWE.
If it's your first reason, then I think it's a bit stubborn of him. The majority of his fans want to see him in the most successful / biggest stage in professional wrestling and headline its main event. He'll let down a lot of wrestling fans in general if he never comes to the WWE just so that he could say that he didn't work for Vinnie Mac. But then again, that's his prerogative.
If it's the latter of your explanations, that is indeed a very sad way to retire. An icon like him deserved a proper send-off and in my opinion the only promotion that can do a wrestling career like his justice is the WWE. While it is true that he has many interests outside wrestling, just disappearing into the mist without warning is another smack in the face of wrestling fans. But as before, that's entirely his prerogative.
Steve Borden is a very private man. Whatever reasons he decides to give or not give in public for not joining with the WWE, I just get the impression that it won't be the full story. But what I really do feel is that WM 27 will be the last window of opportunity; if we do not see him in a WWE ring by the end of the show in Atlanta, I feel that however reluctantly they are willing to do so, fans must abandon any expectations of a deal going through.