I think it's because by 27 these artists have got the money and people around them to indulge in increasingly destructive habits. If they don't get a handle on themselves, it's when the habits get the best of them. If you make it past that, you have probably figured out how to manage your habits. Still weird how often it seems to strike at 27, not 25 or 28. Wikipedia says there is no statistical spike so maybe we don't remember the people who died at 26. If you make it to 30, I think you're out of the woods.