It's more because of the fact that it's engaging, nostalgic, viral, social, and fun to play to boot, more so than any of the reasons disclosed in this post
There have been many engaging, nostalgic games. Viral is just another word for popular. Social games have been tried out before and never really hit off. Still some people feel awkward playing pokemon, I speak of experience when i walk around and point out pokemoners and say, "pokemon go! yeah!", they crinche as they were embarrased. But it might change at some point.