You see how they have somewhat fixed Wyatt. Made him win that feud with Ambrose, he has beaten uppercarders on Raw consistently, good showing in the Rumble, hell they even gave him the coveted mid-Rumble promo!
Thats how they are making him look slightly like a threat after fucking with him in 2014 with all the hopeless flip flopping. If someone like Ambrose charged in with momentum (and by god he has some momentum already) and faced Sting, he would still come out of it better as a loser than in this musical chair IC match we are getting.
Him painting his face and stalking Sting, looking for Sting in the rafters, he didn't need to win as long as they gave it their all in that ring. I stress, this isn't a championship match, you can come off way better than you come in, and it doesn't matter that you lost at Mania. There has to be a great story and a smashing match, the crowd would ne emotionally drained by the end of it and all this was possible.
Don't get me wrong here. I would totally dig Sting vs Ambrose/Wyatt, however, I am just pointing out what the WWE thinks with regards to their bringing in Sting after so long... and their continued belief or 'attempt' to put HHH into the discussion of "Best of the Best" with the likes of Rock, Austin, Taker, HBK, etc.
As for why Taker vs Sting couldn't be done this year. I think after the negativity around Taker(given the poor match due the concussion), the Wyatt-Taker programme is merely an attempt to show that the Deadman can still go and last year was an exception and not really a 'confirmation' that Taker is past his sell-by date in the WWE.