>>149764833
Because South Park makes fun of both sides pretty consistently. While Matt and Trey are very left leaning. Usually for every two jokes at the expense of of someone on the right, say Trump or Mel Gibson, they'll have at least one at the expense of someone on the left like Bill Clinton or Alec Baldwin.
Family Guy, being a product of the early 00s and being at the height of it's popularity during the war in Iraq, usually only makes fun of conservatives, and when they do mock the left, it's usually a more general criticism like 'political correctness sucks' than an attack on a particular leftist actor or politician.