Pretty much story related. If you manage to beat the whole game without getting 50 survivors then you should be in Vegas, not here.
There is a glitch in this game that can be used to your advantage. If you are in a tough fight or just don't want to finish a fight you can quit to title, and continue your game to get back to The Farm and still have all your survivors. The game won't save until you leave and come back to The Farm. This can be done by selecting The Plains and holding left until you reach The Farm again. Later in the game, this is crucial to not losing a single survivor. I farmed almost all my survivors from The Plains since you can smash through these enemies fast and easily.