I'll give you the reasons up front and explain them later
1) Barrier to Entrance Is very Low
2) Gender Unbias
3) Success Breeds On success
Low Barrier to Enter
For any Game to be successful, this is one of the most important factors. As Mark Rosewater points out in this informative blog post barrier to entry can break a game. Who are games made for? This is Highly Generalized. But Truths can be found in generalizations
Men
60 year olds-yes, to interact with their grandkids
50 year olds-no, they're too busy providing for 3 children and a spouse (divorced or togethor).
30 year olds-no, They've just started a job and are plannig the rest of their life.
20 year olds-yes, They are still irressponsible, but often times are thinking about college and
10 years and younger-yes, they like to have fun
For Women
60 year olds-yes, to interact with their grandkids
50 year olds-no, they're out relaxing now that the kids are finally out
30 year olds-yes, possibly new born to show their children
20 year olds-no, Shopping, boys
10 years and younger-yes, they like to have fun
Right off the bat we see most games are centered around children. And If children are having a hard time, theres a problem. Flappy Birds is Extrodinarilly Simple, and in that way allows anybody to play.
There are sites with good statistics such as this one. However, we aren't looking at who plays games now, we are looking at who plays games generally. And Most people start off as children and either keep playing or eventually stop.
Gender Unbias
The Game is gender unbias, the protaganist could be male or female. But more importantly the game is not centered on roleplaying heavily, nor on twitch reactions. Its not based on keeping up with difficulty increases, nor is it playing off beauty and art.
The games art is playing off Nostalgia, where we were all kids once. Memories of super mario world. And Gives you a simple protaganist as a bird that could be male, could be female or even could be both or neither.
The games play is actually very similar to that of Dance Dance Revolution. Where you are told ahead of time what motion you must make, and so when you reach that point, you make it. It probably wasn't planned that way, however any timing, music or dancing is generally popular with women while twitch reactions are more male dominated. The Fast movements making it fun for males though as its like handling a bronco. You can't just drive a plane through the obstacles. There is a proper way to handle it.
Its also masked by a physical area which makes it appealing to children. It makes sense, like a maze makes sense.
Women in general are an untapped market. And when theres a game out thats simple and cute, thats kinda fun, it means that women might buy it. Also Important to keep in mind, women talk to eachother more than men. And If one parents kids are playing something, they are very likely to share it with others.
Success Breeds On Success
When you think of of the name Ugg I think of "ugly", "tiresome" and "dumb"
However, a name like Ugg becomes memorable when its everywhere and everyones wearing it. As Such, its no longer just a name. Its a lifestyle.
In the same sense, viral explosion becomes viral. But what makes somehting viral? A Gaming Magazine? People following a particular Developer? No, its people communicating. Celebrities are interesting in that if they are doing it, other people follow suit. Likewise when freinds are doing it, they they want to try it out also. Thats how I got into PunchQuest, otherwise I wouldn't care.
Then as things get bigger, more reviews come out. More people make media to Parasite off the initial success. And As More Media talks about something, it gets even bigger.
Flappy Birds in all is not a story of solid game development
Its a story about people and how what entertains ourselves isn't necessarilly best for everyone, however what can entertain everyone can be a huge hit
