Because the fight was not conducted under agreed-upon rules, even though it still happened. For example, if someone misses weight, they can just cancel the match. They don't treat it as a DQ win for the other fighter. But when you find out AFTER the fact that the rules weren't followed, it's too late to call off the match. The best thing you can do is treat it as though it never happened, which is exactly what a No Contest result means.
Perhaps that's not how it works in the Olympics. But in the UFC, a DQ only happens for fouls inside the cage, not outside of it.