The Eidos Montreal Blog answers a reader-submitted question that requests an explanation of the decision to make Deus Ex: Human Revolution a prequel, rather than a sequel. Here's their reasoning:

Yes, flexibility.

The five main points we considered:

1) With a prequel, playing the previous games is not a requirement. It gave us a lot more freedom about how we connect to the established cannon.
2) We didn’t have to deal with DX1’s or Invisible War’s multiple endings.
3) Being a true sequel would put us after Invisible War and a bit too far into the future.
4) There was a lot of untapped material in the timeline before DX1.
5) Exploring the time surrounding the introduction of mech augs just struck a chord with us.