Almost whole Europe is habitable for large and dense human populations — there are no serious deserts, no rainforests and shrublands, and only a little taiga.
Almost entire land of Europe allows intensive farming (at least since deep plowing was invented in medieval era) and, in result, high population density.
Without any natural barriers slowing constant migrations, all cultural centers in Europe were connected to each other long before industrial era, which allowed faster exchange of ideas and technology than in any other comparaby large area of the world — maybe except China.
Maciej Kania. Works at Silly Menial Work