The simple fact of the matter is that a lot of theaters really do take the age requirement seriously. Hell, I still remember trying to go see Hard Target with my friends when I was 12 and not being able to get a ticket because an adult wasn't with us. We had to go see Only the Strong instead.
Video games in the mid-90s were still largely looked at as children's toys. The children of that era had not yet grown up into the adult gamers of today, so of course the movie was made in a way that kids would easily be able to go see it.
Mortal Kombat ended up making $122 million on a (extremely well-utilized) budget of only $18 million. I can guarantee you--yes, I guarantee--that it would not have come close to that box office take if it had been rated R.