Comparably, American Football demands a lot from the players. It’s very strenuous physically.
Participants get injured – minor or major – which requires time to heal. So, for recuperation, players are given more offs between matches. This is why there are so few NFL games per season. So that the high-demanding matches do not put their physical (and hence psychological) health at jeopardy.
Aside from this rather assumptive answer, there’s something much definite. After all, ice hockey, too, is equally physical, if not more. Yet, viewers are delighted with more matches. In theories, American Football is played on the high-friction surface. This causes a wear-tear in the tissues of ankles, feet, knee, and calf. Although this isn’t a major thing that would stop players from participating. However, if exposed to such wears with no proper healing, it could lead to long-term damage. On top of that, players cannot avoid such wears with, say, extra padding's. Because it has to do more with the postures when the players run and jump and fall. When done awkwardly, which usually happens due to the high-friction surface, the players sustain wears.
Now, indeed, you would think changing the type of surface on which American Football is played can solve this problem. However, that’s a no-no option. These green surfaces are designed and prepared so to minimize the impact of any foul landing or tackling. Changing it can result in much more serious injuries like cracked neck, broken ankles.
So, in short, there are so few NFL games per season because it provides players to recuperate properly from physically demanding matches where they often sustain subtle injuries.
That being said, I don’t mind fewer matches really. It gives an exclusivity to NFL and makes few matches more anticipating. The wait till your favorite team plays is the best part.