When people slowly trickle in, they get to know the culture of the place, they learn the people and personalities involved and can decide how or even if they want to fit in. When you get a nice slow steady growth curve, you maintain the site and build a community.
For an example of non-organic growth, look what happened to Voat. Voat was a right-wing Reddit clone with the exact same and feel as Reddit, but from a conservative circle-jerk versus the liberal/progressive/libertarian leanings of Reddit. Then last year the "fat people hate" and racists mostly bailed on reddit and overwhelmed Voat in about a month. The whole character of the site flipped a switch and is now not something I want to deal with.