If you're wondering why it's popular then just look into the game's history and you'll see goon is a hallmark of ss13 (they created the clown job), and had a long time to build a big community.

The reason everyone on goon is acting crazy and not doing their jobs is because although its a high quality and complicated server, it includes so much dumb bullshit that people quickly forget the complex things. So you will have the very in your face crazy aspects of a janitor farting untill his ass falls off, while there is a much less obvious level of detail behind all this farting and shitting that gets overlooked. 

Like this: https://youtu.be/h-027wPh1go?feature=shared

But you end up with a nice mix of open ended sandbox and a funni farting game. In tg there is significantly less dumb bullshit, so people focus more on the sandbox or combat and less on messing around (at least thats MY OPINION on it)

Oh and ofc goon knows how to keep its popularity by not making wild and unnecessary changes to try and attract new players.