In my opinion that were the geographical position and large number of great sailors and explorers.
Explanation:
Even in the first half of 15th Century Portugal started spreading its maritime strength, which was their main focus when it comes to development of the country. Of course, the position of the country, that is located on the shores of Atlantic Ocean allowed the sailors from this country to gain certain knowledge. Of course, with the technological improvements of that period they started sailing on the open sea. Of course, all of this wouldn't be possible without great sailors and explorers that opened the Age of Exploration. Among them the most famous were Dias, Vasco de Gama and Pedro Cabral.
