Once something get's a bit of momentum, every you-tuber and blogger has to write about it (coz it's cool). Cool = hits. How it started I don't know, but the online media is responsible for it's continuation. Once something is in the headlines, there are 20,000,000 people that have a social duty to know what it is. Anyway, that's my cynic's perspective. And it cost's nothing to check it out ( I just did ).
Most frustrating game ever, I would say the only reason people are playing it is because it's popular. It also uses Mario graphics, which is the clincher.
Edit : Think of it this way, if the game didn't have advertising, there would be no possibility of saying "Popular mobile game Flappy Bird making $56,000 a day" .. well that would take away well more than half the media attention. Does that mean it's actually the ad at the top of the game screen that's making this game popular. Sure it must have been getting some downloads to start with. But what made it big ? .. The ad !