Because the S.H.I.E.L.D. TV pilot takes place after the battle of New York.
Source: No, it's Not a Prequel: ABC President Confirms S.H.I.E.L.D. Takes Place After The Avengers
LMD? Ghost? Time traveler? Hologram? Immortal? ZOMBIE? Just what is Agent Phil Coulson? Unless Joss Whedon's S.H.I.E.L.D. television pilot takes place in an alternate universe from the Avengers movie, Coulson may not be human. Because -- spoiler space -- he died in the Avengers movie.
The tidbit about the S.H.I.E.L.D. television pilot taking place post-Avengers movie, came during an IGN interview at the Television Critics Association (TCA) press tour. ABC president Paul Lee said the pilot "…is part of the Marvel Universe. In fact, the story takes place after the battle of New York".
That being said, don't expect to see Thor or Captain America starring in the show. Lee said that they will be careful not to "tread on the toes of the features and build a whole new world".
The S.H.I.E.L.D. pilot start shooting next week, but nothing beyond a pilot has been ordered by ABC.