There is some speculation as to why Sting has yet to sign, many of the rumors surrounding the hold-up seem to be the status of The Undertaker. One of the incentives for Sting to join the WWE was the possibility of a match with The Undertaker at WrestleMania 31 and now with The Undertaker having lost to Brock Lesnar at WrestleMania XXX and possibly retiring, that incentive is most likely gone.
Sting has often said he wanted to end his career with one WWE run and allegedly there are select dates for Sting to wrestle including at WrestleMania 31. There was also talk of putting Sting into the WWE Hall of Fame either next year or 2016 but as of now, all bets are off.
The Undertaker suffered a severe concussion during his match with Lesnar and was rushed to the hospital with WWE CEO Vince McMahon in tow. Undertaker reportedly collapsed after returning back stage and could not remember much of what had happened during his match. The legendary figure was kept overnight in the hospital and released the next day and was at RAW but was not used.
Sting has toyed with going to the WWE for the last few years and came close to a deal last year when his contract with TNA expired but Sting chose to renew with TNA.
With the untimely death of The Ultimate Warrior, who had signed to be an ambassador for the WWE after his Hall of Fame induction it is believed now that Sting could fill that role after his wrestling career is over.
While no one is certain why there is a delay from Sting's side about signing the contract it is believed he will eventually become part of the WWE. What his role with the company will be at this point, especially with the uncertainty of The Undertaker, is unclear.