Flappy Bird wasn't overly popular when it came out, it was only later on that it went viral (I think about 6 months or so after it launched).

A big week 1 surge requires pre-existing name recognition.

Games with a 2 in the name are the next most likely to have heavy first week downloads. Angry Birds 2 or Bejeweled 2 for example.

No, flappy bird was a slow burn