Sydney's weather was amenable to having a summer Olympics during their end of winter/early spring. I don't know how beach volleyball players would feel playing morning matches in 65 degree weather, much less open water swimmers, in September. Rio's temps are the lowest during August with highs around 73 and lows around 62.

The reason why they won't allow another Summer Games in the Southern Hemisphere occur any later than September is because it "disrupts planned competitions for later in the season for many sports." That's the reason the IOC gave Qatar for not hosting the 2016 games. Otherwise, apparently Doha was a completely capable candidate against the other bidding cities. It even beat Rio in early voting.

It should be noted, though, that both Tokyo and Madrid received higher scores than Chicago in preliminary voting. Geography aside, as of earlier this summer, Tokyo and Madrid were considered more capable host cities by the IOC than Chicago.