I think the reason why it wasn't explained is because the Avengers movie is supposed to take place after certain parts of certain movies. Like Avengers takes place well after Iron-man 3, I read this somewhere but I don't remember where. Assuming it takes place after Iron-man 3 I think it also takes place after Thor 2 in which they explain his return. It doesn't make sense I know, but hey what can you do right?
I think the reason why it wasn't explained is because the Avengers movie is supposed to take place after certain parts of certain movies. Like Avengers takes place well after Iron-man 3, I read this somewhere but I don't remember where. Assuming it takes place after Iron-man 3 I think it also takes place after Thor 2 in which they explain his return. It doesn't make sense I know, but hey what can you do right?