Personally I have been thinking about this the last days and wondering why, why Pokémon Go became so popular, and not Ingress, which is very similar and also made by the same company!
Some quick background information about Niantic Labs, they started out in 2010 as an internal startup within Google, and later came out with Ingress which got it's hype, but never the real trend as Pokémon Go! Then Niantic Labs spun out of Google in 2015 and teamed up with The Pokemon company in 2016.
Then the adventure began. They used a lot of rally points which where created in Ingress, so they already had all the data points needed around the world. Making time to market extremely fast! That is also why the app works around the world for countries that not yet have gotten access through it via their app stores. Curious to where you can have pokemon Go, check this site
So back to the original question, why did it get so popular?
First of all I have to say that Pokémon is a strong brand name! Everybody knows the name somehow. Much stronger than the Ingress name.
My mother even knows the name Pokémon and she is 65, that is because I wanted some sort of Pokémon toy when I was younger. My younger siblings of 17, 12, 11 knows the name because all of them are updated on the internet! That spans 3 generations. It's quite a brand name!
That would conclude as reason number one!
Reason number two is the moving and getting around part of the game! Yeah alot of people say, screw that, I want to play in my apartment by myself. But MMO's are a hit, so people like to play with eachother. Pokémon Go brings MMO into the real world. That combined with reason one, that everybody knows the name Pokémon, and can easily explain what it's about.
Yesterday I tried to convince a friend of mine to play, and the reason that got him to download the game, was that I told him that I recently stopped at a lure(Pokémon Go thing), and ended up talking to a couple of guys I didn't know. We all realized that we loved kiting, so we planned to kite the next week. That would never have happened on the internet, first of all because most of the players I would play with would be scattered all over, and not located around me.
That would be reason number two, you actually get to experience the real world together with a sci-fi world!
Lastly we have a third reason, that would be a combination of reason one and two, that you acctually catch the pokémons in "real" life. You walk around, see the pokémon near you on the map, hit the pokémon and then you acctually see him infront of you on the street. Of course this is through the screen combined with the camera, but this generates a lot of cool scenarios!
That would be my 5 cents about the popularity of Pokémon Go!