How did Miku become so popular?
Here is an excerpt from: http://kotaku.com/5936200/why-hatsune-miku-is-the-worlds-most-popular-virtual-idol


  Hatsune Miku is more than a little big in Japan. She appears on billboards, in TV commercials, and gives sold-out concerts to legions of fans the world over. Of course, being a popular singer in and of itself isn't something that out of the ordinary. However, what makes her unique is the rather large obstacle she had to overcome in her path to fame—namely that Hatsune Miku isn't even a real person; she's a computer program.


The main fact that she is a program that anyone can use to make their own music despite perhaps not having an optimal singing voice opened up new paths for many music artists. This led to many popular Vocaloid songs done by Miku such as MELT or World is Mine.