The latest rankings of the Top 400 Universities in the World, 2011-2012. A breakdown of their locations by country followed by some explanatory questions:
Top 10:
United States 7
United Kingdom 3
Top 20:
USA 14
UK 4
Canada 1
Switzerland 1
Top 100:
USA 51
UK 12
Canada 5
Australia 4
Netherlands 4
Germany 4
France 3
Sweden 3
Switzerland 3
Republic of Korea 2
Japan 2
China 2
Hong Kong 2
Singapore 1
Finland 1
Belgium 1
What explains this distribution? One is that education and research are very expensive, so it makes sense that the wealthiest countries would have the best universities.
A follow up question: What makes countries wealthy? The great majority are in countries that have significant amounts of economic freedom.
Wealth and economic freedom are necessary conditions, but great universities also need intellectual freedom and other cultural traits. Another correlation: 75% percent of the top 100 universities are in countries with strong historical connections to the British empire. Most of the rest are also in cultures of political, religious, and 
other social freedoms.
[And something for the egalitarians among us to worry about: Note that Canada has only half of one percent of the world’s population (34,108,752 of 6,840,507,000) but ten percent of the top thirty universities in the world. How disturbing. Clearly the crafty Canadians are hogging a disproportionate amount of the world’s knowledge resources.]