It don't works like this. Success come from a lot of factors and you can't just say that it's because of this or that.
There's the arts, which is effectively strange, but relatively pleasant for the eyes. I don't say that it's beautiful, but its look-like childish book side make it easy to deal with it for hours. This while there's 2D games with amazing art that become tiring with time and need that you make a pause after some times playing.
There's the free roaming side, like said
@Joraell , which add something to the game. Plus the story, which is basic but interesting and well built, at default to be effectively well wrote.
Then on top of this you need to put the fact that the game have years of existence behind it, which obviously make it know from way more people. Plus the fact that it was one of the first of his kind, which give it a benefit in regard of the others. It's not the only not badly done free roaming 2D game available, there's also
Lust and Power by example. But like it's the oldest, it's the first one that come to our mind when we have to name a game like this ; and so the one we name the more often, which imply the one that have the more mouth to mouth advertising.
It's all of this that made its success, not just one of these points.