Even before Sting left TNA this January, the word on the street was that he would sign with WWE this year.
Given those longstanding rumours, a deal was expected to be struck quickly, so he could appear in some fashion at WrestleMania 30, but surprisingly WWE didn't see it as a top priority, likely because they felt they had enough star power for the event already in place between Batista, Hulk Hogan and The Ultimate Warrior's returns, cameos by Stone Cold Steve Austin and The Rock, and the usual cast of headliners (minus CM Punk).
Apparently, WWE were holding back on Sting's debut because their working idea was for him to do one last match at WrestleMania 31 next year, plus it would be stupid to start building anything without a firm commitment in place.
A few days ago, Dave Meltzer reported that Sting had finally signed with WWE, but now it looks like he jumped the gun on that story.
In his daily news update today, Meltzer clarified that WWE are still waiting for Sting to put pen to paper after sending him the contract he wanted:
"An update on a story in the current Observer. On the Sting/WWE deal, WWE sent Sting his contract several days ago, but as of yesterday, he had not returned it. The belief is they were on terms and an agreement is in place, but as of yesterday, pen to paper on Sting's side hadn't been returned. There is no indication of any serious hold-up at least from the WWE side. Those in the company even before WrestleMania were working with the impression he was on board."
In the end, I expect any problems to be ironed out long before WrestleMania 31, but the timing of this is interesting, coming days after Brock Lesnar ended The Undertaker's WrestleMania streak and what is now presumed to be The Deadman's last match ever.
Sting has made it abundantly clear that The Undertaker was the only match he wanted and it certainly seems that WWE at one point led him to believe he would get that dream match, but now that's probably off the table, which could explain his delay in signing the contract. It would certainly make sense that Sting would want some reassurances that WWE has got a good replacement opponent in mind before committing his future to the company.