Basically: because it has a lot of characteristics in common with all the other, better preserved, asteroid craters we know of. So our hypothetical advanced civilization would have to have deliberately mimicked all the little details of an asteroid impact, not just created a big explosion using some other method. That's possible, of course, and if they did it well enough we'd never know. But it seems much more plausible that it was an asteroid impact.
saratogacx|7 years ago