It's the same reason why teams in the NFL down by 20 don't use all their time outs in the final 2 minutes to get the ball back. The game is over.
No, probably wouldn't have medalled, but I think a lot of people, the above poster included (who you so blindly judged), wish she would have at least finished. DFL > DNF We'll never know what went through her mind in both fall scenarios, but I think a lot of people have a hard time understanding why her instinct was not to get back up and chase.
Her instinct was to chase - she chased her competitors a year earlier when there was some hope as there were more than two laps remaining but even then she ended up just 9th even though she was the world leader and world #1 that year.