Thats a complacated question. I think the reason we 'allow' this is because its not even on most peoples radar. We have become so "busy" with day to day problems like having a home we can afford, 2 or 3 cars so everyone can work, and the things our kids 'need" that we just don't take the time to really really look at things like this as a 'problem". We have built ourselves a nice little universe and if the suppernova's don't directly affect our 'planet' we're just not going to worry, even if we know in the back of our mind that someday one may destroy our 'planet". I don't fault people for this, its an ingrained reaction. We deal with what we have to on a day to day basis and this has allow great advances in culture along with great problems. Now the fact is we have so many problems facing us that we might feel helpless to change all the things that have been creeping slowly to a head.