1. They're all on the same flight because it's possibly the most convenient one for reaching Melbourne.
2. The conference is for 25000 delegates from all over the world, and has been running since 1985 when the first one was held in Atlanta. See here for a list of cities that have hosted it : http://www.aids2012.org/WebContent/File/History%20of%20the%2...
Nobody factors aircraft accidents or intentional shoot-downs for conferences like these, for the very reason that air travel is so ridiculously safe that it is pointless adding the additional logistical cost to do so.
I didn't realize it was a conference in which about 25000 people participate. Then losing 100 experts all of a sudden sounds much less critical to the research field than the article suggests.
It is usually a few professors, who have their researchers and a bunch of PhD's with them. The thing with researchers though is that if done properly every single one of them should be a leading expert on a very specialised area of the field. Only the full professors probably could more generally be considered a leading expert probably.
Nobody factors aircraft accidents or intentional shoot-downs for conferences like these, for the very reason that air travel is so ridiculously safe that it is pointless adding the additional logistical cost to do so.