The difference between fighting games and other genres is that they are harder to balance out. For shooters for example it's simple. Give someone a few different guns, a few different enemies to shoot at in different environments, and you have a shooter. For multi-player it's not a big deal because everyone can use all the guns etc so the balance is probably easy to find. Might be a mediocre game, but might still be fun.
With fighters it's another story. Fighters need optimization to be good. And developers that have no experience with fighters might have a very hard time creating a good game from the get go. Since fighters revolve around different characters with different styles, they need to find a way to make every character balanced. For example, a slow yet safe and high damage character and a fast but unsafe low damage character, or anything in between. The strings must have enough mix-ups, and animations are important. Besides that, there are other stuff as well, will it be 2d/3d? What influence do environments have on the gameplay, how fast should it be, what about launchers, juggles, ground games, stuns, charges, block breakers etc etc. Though fighting games seem simple, they are one of the hardest games to develop properly.
Besides that, fighters always have a steep learning curve compared to other genres.. Since every character controls differently it's harder to master than a shooter for example, or an rpg since those have standard strategies tied to them.. Most people will forget about fighting games because they get slaughtered when they go online and playing on yourself isn't so much fun, and it's hard to get good. So in the end you'll end up making a game that costs a lot of effort and only a relatively small group of people will really get into it.. So that's why fighters are less common than, let's say, the beginning of the gaming industry.
Truth does not fear investigation
