That's a common misconception about the character. People look at the wrestling aspect and immediately mistake it for a Fighting Type.
However, having the basic understanding of what the Dark Type really is should clear up why Incineroar has always been a Dark Type from conception to execution. Dark Type, known as Evil Type in Japan, isn't about literal darkness or the occult, but instead about being mean, dishonorable and/or underhanded; think of it as the jerk or bully type. The moves that are given Dark typing are common cheating and underhanded tactics. You have moves like Fake Tears, Knock Off, Sucker Punch, Embargo, Feint Attack, Etc that are all about deceiving your opponent to give you an unfair advantage. Then, if you look at other Dark Type Pokemon, you get things like Scrafty/Krookodile/Pangoro which have a street punk motif going for them, Greninja which is an assassin, Shiftry which is based on a Tengu spirit, Tyranitar which is a violent Kaiju, and so forth. In general, the Pokemon of this typing tend to have a more malicious appearance, or in the case of Absol, perceived to be malevolent.
On the other hand, Fighting Type is about being fair, honorable and athletic. It's often associated with heroic ideals as well, namely in the sense of the Musketeer-inspired Pokemon from Unova, where they fighting type may not make as much a sense since they don't directly reference martial arts.
Incineroar is the Heel Pokemon, which it inherited from its inspiration, the classic Manga, Tiger Mask (who originally was a heel himself). For those not familiar with wrestling, heel is the term used for a antagonist or villain character who's role is to generate anger and be hated, often times by cheating, being overly violent or just a big jerk, so that when the hero character, or babyface, opposes him, the fans will cheer on the good guy. Combine this with the fact that Pro Wrestling is a scripted/acted production and not a true athletic event, and the Dark Typing is a natural fit for Incineroar.
Sounds about right, but I felt like it would help those who may be confused about Incineroar's typing to have some understanding as to why it's Dark and not Fighting.