Chicago was a favourite because it was in the US... or rather it was a bigger favourite than Tokyo and Madrid because of that. The US means money like it or not.
However saying that, most media in the US and even in Europe have never understood the IOC. They are the ultimate old boys club, full of former and present royalty and political big wigs. So lobbying by famous people is their bread and butter.... it is never the actual facts of the bids that win the day.
And if you look at the different city's presentations as I did, it was pretty clear that Chicago and Tokyo would fall first since their passion for the games was no where to be seen during the presentation. During the Chicago presentation only Obama managed to drive some passion into a very average presentation.
On top of that, Chicago had some big issues with it. First off the Atlanta games. They are seen in a relative perspective, as a failure to live up to the Olympic passion. I have heard comments of Atlanta being more like a county fair than an Olympics. Secondly, Salt Lake City.. the corruption Olympics. It is not forgotten in the IOC by any means. Also the US has had more of its fair share of Olympics.. by far the most for any nation. And lastly, there was questions on the financing and some venues.
Tokyo was always the underdog, since it lacked the passion many IOC members want from the start. Plus a huge hurdle was and is Beijing as it was just held in Asia. No one doubted the financing nor technology aspects, but there was a clear lack of Olympic passion.
Madrid was also always one of the dark horses. Not only did it have 75+% of the venues already done and by far the best transportation system of the 4. On top of that it had the Samaranch factor.. you can not disregard the former Olympic Presidents pulling power in the IOC. But against Madrid... London 2012. Never since the rotation principle was put in place has a games gone to the same continent in consequent Olympics. This is a huge hurdle, even though on paper it was the best bid.
And finally the winner.. Rio. Biggest bonus... it is in South America, a place the Olympics never has been before and the IOC being a bunch of old conservative wankers who love a good "story", that would also be a huge hurdle to get over. Despite having the biggest budget and by far the biggest social problems on the ground, this factor in the end won the day.
Brazil and Rio have a very long way to go. Rio is crime infested, lacks a transportation system and many of the venues are still only a piece of paper. And to prove that Rio winning was based on a "good story" rather than facts.. Rio has to hire many cruise ships because there are not enough hotels in Rio...Rio had by far the least amount of hotel beds of all the candidate cities. The 14 billion dollars put a side.. will not be enough.