They were basing it on what modern exoskeleton-suits look like. Most of the technology utilized by the people on Earth in Elysiun is stuff that you would likely see in the next 20-30 years. So a slimmer, non-tethered, less power-consuming exoskeleton is going to be developed eventually- and it may end up looking like the suit in the movie.

I just think the design makes sense with the context of the movie, even if it doesn't look that cool. Besides, Kruger, the villain of the film, has a sleeker, newer model of the exoskeletal suit. So there's a contrast between Matt Damon's shitty suit, and Kruger's awesome suit that makes him more intimidating.