Olympic game host cities are allocated based on the merits of the application and how recently the nation has hosted a games. That is to say, a city will not be allocated a games if the country that it is in has recently hosted a games.
Since the allocation is based on country and not continent, the fact that European countries have hosted more is of little relevance.
A simple factor may be that Europe has more countries with a suitable climate and the capacity to host a game than any other continent.
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Australasia/Oceania have hosted the Summer Olympic Games twice. These Games were in 1956 in Melbourne, and 2000 in Sydney.
Europe, Asia and North America have hosted the Summer Games on more than two occasions each.
South America is due to host its first Summer Games in 2016, whilst Africa (and Antarctica) have never hosted one.
North American has not hosted the most Olympic Games ... Europe has by almost 4 to 1.