Nitrogen and hydrogen make ammonia. The forward reaction is exothermic, and the number of gas molecules drops from four to two.
So lowering the temperature and squeezing the vessel both move the balance towards ammonia. You can check each of them by the step in the graph.
But lowering the temperature also slows both directions. The balance ends up in a better place, and it takes far longer to get there. The Haber process runs hot and at high pressure precisely to settle that trade-off.