There is no rotation of continents. No Olympics has been staged in continental Africa - in fact there has only ever been one African bid (Cape Town) for the 2004 Games.
Statistically US cities may host a 'disproportionate' and 'more frequent' number of Summer Games because they make up the majority of bids. Of 169 bids to date, 36 (21.3%) were by US cities. The next highest tally are Italian, Spanish and German cities with 9 each. Los Angeles has put in the most bids of any city (9), followed by Detroit (7) and Rome (7).
London's winning re-bid for the 1948 Games (having originally been awarded the cancelled 1944 Games) was against Lausanne and 4 US cities (Baltimore, Los Angeles, Minneapolis and Philadelphia).
Helsinki was awarded the 1952 Games against bids by Amsterdam and 5 US cities (Chicago, Detroit, Los Angeles, Minneapolis and Philadelphia).
Melbourne 1956 won against bids from Buenos Aires, Mexico City and 6 US cities (Chicago, Detroit, Los Angeles, Minneapolis, Philadelphia and San Francisco).