I have my older brother to thank for getting me into one of my now-favorite shows. Much to my mother's chagrin, it's considered highly inappropriate, and she's annoyed whenever we turn it on. While she's not a fan, it's my go-to when I just need something to watch.
"It's Always Sunny In Philadelphia" is nowhere near being one of those family-friendly sitcoms with a tight-knit group of friends that get through everything together (such as "Friends" or "New Girl"). It's basically the complete opposite. The Gang consisting of Mac, Dee, Dennis, Charlie, and Frank constantly focus on wildly inappropriate topics while fighting with each other in every episode -- and that's over 150 episodes now. It's hilarious for someone with a twisted sense of humor but others think it goes just a little too far to be funny.
It's the same concept for a show like "South Park" (which my mom also does not like). Everyone's kind of terrible and inappropriate at all times. But shows like "South Park" and "Sunny" do something others can't -- they go to places others won't because they've already broken through that mold.
When you've been doing your own thing for so long, you're not going to go back and conform just so more people will say your show is funny. These shows already have plenty of loyal viewers and do well enough with what some would say is lowbrow comedy. The subject material may be vulgar at times, but it's so smart -- other shows can't make you laugh and question gun policy all in the same episode, and most wouldn't even try.
I for one have accepted my dark sense of humor and have enjoyed each and every episode of "Sunny." I was fully prepared for season thirteen to premiere and am so happy to see my favorite dysfunctional bar crew again.