What are some reasons for why the movie Super 8 takes place in 1979 instead of 2011?

I have not seen the film yet but here is a snapshot from a review that addresses your question. Basically it has to do with the director trying to capture the innocence and magic of classic Spielberg coming of age films from the early 1980's.

J.J. Abrams Super 8 is an echo. It echoes the innocence of Steven Spielberg’s Amblin films of the 1980s, it echoes the imagined purity of small town America, and it echoes the innocence and coming of age through the lens of aspiring filmmakers. But Super 8 never makes its own noise. While the film manages to capture the fun, adventure, thrills, laughs, and characters of Spielberg’s movies, Super 8 never conjures its own magic....Super 8 is a loving homage to the early directing and producing work of Steven Spielberg. Abrams struggles to evoke a feeling rather than making direct references to E.T., The Goonies, etc. It’s an honorable goal and I respect Abrams for attempting to make a film that stands alongside those beloved movies.