Corruption played a big part, but Sochi had zero infrastructure at the start, it all had to be built from wholecloth, and Beijing need to transform large parts of an existing city to become an international showpiece. London already has incredible infrastructure and is already iconic. It seems that the strategy the London Olympics really used was to simply extend off of the existing systems in place and build up in a cheap area bringing new economic development to that area instead of trying to retrofit the city core.