Because MLP is made in such a way that everyone, not just kids can enjoy it. The show is filled with adult jokes, the punch being "spiked" immediately springs to mind, and fluttershy covering herself up. As well as references to popular culture, both in the background and foreground, such as; "winter is coming everypony", Pinkie's Bat signal, the Futurama pony, Kratos from God Of War, the Dragonball Z style laser battle between Tirek and Twilight, TRON, Slenderman, Weird Al. There are loads more, but those are the more well known ones.
These are things that little girls obviously aren't going to pick up on, as they are more geared towards teens and adults, which make the show more enjoyable for us, autism or not. Plus the show is written in such a way that adults can enjoy it too, the whole point of this is to be able to watch it with your kids.