Where did you get the idea that this is supposed to be set in the past? I can't remember anything like that being said in the series. As far as I saw, they seem to have some significant similarities to our world (notably the worship of the Greek pantheon), and a whole lot of similarities that are there because the makers had limited time and budget, and they chose to spend it on decent scripts instead of exotic clothes.

You can see it as a psychological feint, if you wish. It's much easier to see somebody as an average person like yourself if they're dressed pretty much as you are than if they're dressed in silver spandex. And if you believe that they're people like you, their fate becomes much more engaging. Speaking for myself, I think this series would've been much less interesting if they'd spent the effort to make up clothes, uniforms, furniture, tech, control structures and everything else that now is pretty much like ours.