Because its junk.
Really, there is no explanation for it. Why would they build a test car w/o airbags as stated by the Jalopnik site? Isn't that the point of a test car, to make sure
everything works properly? Hyundai is gonna get boned for this one, especially because it was a very significant impact and by anyone's standards they should have gone off. I have seen airbags go off from a much less brutal collision.