Really interesting article, one thing I thought fascinating, was the idea: we and gorillas (and chimps) share a common ancestor that developed complex societies, but chimpanzees have lost a lot of this trait. It makes we wonder what was the stimulus that caused their genus to undergo a regression of social skills?
Also, the "Darwin" hoodie the Dr. Morrison is wearing at the bottom is awesome.
By the reasoning of who you're replying to, the answer to your question would be that it's because we share the same common ancestor with both chimps and gorillas—gorillas also inheriting the improved social skills that humans have, making chimps the odd ones out.
Which is a leap - maybe our common ancestor had something which bloomed in gorillas and humans, but not in chimps. It's not necessarily the case that the chimps have reverted.
Also, the "Darwin" hoodie the Dr. Morrison is wearing at the bottom is awesome.