Have you played a Tecmo Super Bowl game recently? It's not just nostalgia that keeps people playing it.
The formula still holds up incredibly well as a head-to-head experience. I'd go so far to say that it is the greatest form of head-to-head football to date.
Since it's simple, anyone can pick up the game and play it, unlike Madden or All Pro, which are very frustrating for beginners and take weeks of practice to understand the finer points of the game.