Thinking from the perspective of a reality show, it would make a lot of sense to observe our species just as an experiment. Now thinking about the planet and all the other species that live here, it wouldn't be very wise of them, unless life in the universe is not as scarce as we think. Another hypothesis would be... what if aliens wanted to use our planet? So I believe they didn't let it get to that point, because the planet would be doomed for years and years by the amount of radiation.