Agents of SHIELD: Season 3 - Discussion (SPOILERS LIKELY)

Coulson is the epitome of a breakout character. He was just a minor supporting player in Iron Man, but Clark Gregg brought such charisma to the role that the fans and producers wanted more, so they gave him an enlarged role in IM2 and Thor, made a couple of short films centering on him, then featured him even more in The Avengers. There were fans asking for a Coulson TV spinoff even before The Avengers, I think, and the clamor intensified afterward.

 

Sorry to dredge this up again, but I just happened to be re-watching the episode "The Inside Man" and found it to be the source of this being established. Hive tells Malick: "We cannot feed on our own kind."