Properly conveying the size and behavior of a shambling horde of mindless, slobbering zombies takes up a lot of memory in video game development. That's why the crowd at Cleveland Browns games is even more lifeless in Madden. And it's why Dead Rising 3 decided to sit out the current generation for more power in the next.
The original Dead Rising couldn't have more than 1,000 zombies on the screen at one time—which sounds just insane when you think about the kind of frameskip you can see, to this day, in a sports title with a bunch of players and sideline actors.
Dead Rising 2 pushed that, and Dead Rising 3 will almost triple what you saw in the last game, but, in an interview with Siliconera, the games creators realized there was no way the Xbox 360 would be powerful enough to accommodate an expanded horde. (Dead Rising 3 will be an Xbox One exclusive.)