Does anybody else not understand why wwe dont have John Cena vs The Undertaker at WM 27?

WWE are probably going to have Cena vs Punk and maybe Kane or Barret vs Taker at WM. Now this could be Takers last WM so why not have Cena vs Taker. They did it with Michaels & Taker at WM 25 & 26 so why not have this at WM 27. Now I know Taker is injured and might not make WM but would you not at least leave the match open. Cena does not have to turn heel he just has to say that he wants to beat Takers streak. Can you give me a reason other then the reasons I have put up why Cena vs Taker is not happening at WM 27.