It's because it was shallow bullshit.
"Mature" R-rated animated features don't exist, simply put, every R-rated feature film is just lots of swearing, gross out humor, sex or gory gratuitous violence for the sake of shock that ends up being less mature than animated films with kids as the target demographic>Fuck shit motherfucker>Guys, my entire motivation for everything is to get laid, because I'm a guy and that's what being a guy is all about, right other guys?>Seth Rogan yelling and telling the audience what disgusting thing is happening right now.>DUDE DRUGS LMAO XD>SEX
This is the shit that edgy tweens think being mature is, any grown adult will recognize this trash for what it is which leads to an ironic situation where the movie's actual demographic isn't allowed to see it and those old enough to see it will see it as mediocre at best.
Fucking Lilo & Stitch is more mature than Sausage Party, dealing with heavy themes of loss, moving on and redemption with nuance, with the plot boiling down to "Criminal created to be the worst thing in the galaxy escapes and crash lands on Earth and learns to rise above himself through the concept of family while inadvertently helping a little girl come to grips with the loss of her parents and the lack of understanding her older sister who now must take the responsibility of raising a child alone".
Shit like that is way more interesting than a frat douche's quest to get laid written by boomers, not only that but it can be appreciated by everyone and not just sixth graders at the height of puberty