In The Martian, I understand that all the bacteria would have died after getting exposed to Mars' atmosphere with sub zero temperature, but Mark's poop will always have bacteria to cultivate more crops. So why did he stop growing more crops?
Because he lacked the resources to start over. All the plants/potatoes were dead from lack of oxygen and sub freezing temperature. The bacteria in the poop soil as well. And his source of fuel that he could turn into water.
So he had nothing to plant, in soil that was sterilized, with no water to sustain it. The potatoes are no longer suitable to plant, just to eat.
Of the three, maybe he could start over with his own collected poop or some left over poop, and maybe he had fuel left, but without any living plants or seeds to start with, it's a no go. And even then, it's a question of yield. With very limited supplies, all the work he would put in to start over would not produce enough for him to survive on. His initial setup was already small, but with enough supplies to live on for months. After the accident he would have slim to none. A single potato and a single human poop factory is not sufficient.
As the books put it: Potatoes are now extinct on Mars.
Side note: The book explicitly states that the bacteria survived the accident, and was growing, so it's really down to not having anything to seed.
You are making a good point. In terms of how the science was presented in the movie, there appears to be nothing stopping him from starting over. But here are some possible things to consider:
- The great amount of effort for a full restart will only make another mishap all too possible to justify the effort.
- Other crucial activities for survival put a full restart lower on his list things to do to not die.
- Mark has a unique ego that drives him. Perhaps being the type of man that is "the best botanist on Mars", a "space pirate", and the man who "goes Ironman" mean he will not pursue what he perceives as a failed strategy.
- The catch all: That's what will make the movie plot move along.