There are a number of reasons which make Pokemon Go popular. I believe that the Ingress User base plays a key role, that marketing had an effect and that curiousity also played an important role.
Big user base
Ingress has a large and vibrant community of players. These players are so passionate about this game that they are willing to walk 30 or more kilometres a day whilst staring at their phone. They are ready to climb to the peaks of some mountains and brave rain, sleet, snow, high winds, mud, heat and the cold. How many of us have had our fingers go numb as a result of playing ingress after all. That passion for the game was seen by children and Ingress playing parents let their children play. The children found it fun and so when they heard that Pokemon Go was coming out they automatically wanted to use this game. This adds a second dimension. Parents who enjoy playing Ingress are likely to have more than one mobile phone and more than one data sim. These parents are ready to give a mobile phone and sim card to their children. These children are then likely to play with friends. Yesterday evening when I was with Ingress players they had already switched to Pokemon Go and they kept commenting on the number of children they saw already playing the game. Remember that we live in a globalised society. Whether you live in France, Belgium, Germany, Australia or the United States you will have heard about Ingress, about Pokemon Go and you will have been curious to try the new game