I think it's a combination of its difficulty, strangely endearing pixel graphics, and the quirky name. The collision detection is a bit off, but otherwise the controls, scrolling and animation are pretty crisp and smooth. The strangely addicting and downright infuriatingly hard game play compelled users to write "interesting" app reviews, which turned into an ad-hoc contest of writing the funniest review and posting it to Twitter, which went viral. The rest is history.
If the current income holds, that Vietnamese guy is set to make $18 million per year. Good for him.