#3 Unfortunately, this isn't 1999...
...or even 2015, when both Sting and The Undertaker were capable of producing a good enough match. At WrestleMania 31, The Phenom looked in incredible shape and his match against Bray Wyatt certainly wasn't a bad affair. On the other hand, Sting held his own against Triple H, although the bout was marred with interference by DX and NWO.
Also read: 5 biggest reasons WWE can't leave the PG Era behind
Fans and wrestling personalities have been saying this for a long while now. The Undertaker is way past his prime, and so is Sting. If by a stroke of luck and a good enough pay, WWE had managed to score Sting after buying off WCW, it would have been the perfect time to greenlit the dream match. Sadly, it didn't happen. There was an opportunity to do the same at WrestleMania 31, but the company decided not to go ahead with it.
Now, 4 years later, things have drastically changed and there's no logical reason to have this match.