That's because you're not mature enough. The humour in South Park is very clever and mature while still maintaining an immature appearance to those that don't frequently watch it. Like for example, the economy episode which was the very first episode that I watched and the episode that got me liking it; the things they talk about are things only clever, smart people would understand and details the current world affairs around us. Americans being the dumb shits that they are mostly prefer Family Guy because of it's incredibly dumbed down humour.