The reaction becomes slower as the temperature becomes lower.
You need the gases to reach equilibrium within the very short time that they will be in contact with the catalyst in the reactor.
400 - 450°C is a compromise temperature producing a reasonably high proportion of ammonia in the equilibrium mixture (even if it is only 15%), but in a very short time.
Pressure
Increasing the pressure brings the molecules closer together. The higher the pressure the better in terms of the rate of a gas reaction.
Very high pressures are very expensive to produce on two counts.
You have to build extremely containment vessels and strong pipes to withstand the very high pressure. That increases your capital costs when the plant is built.
High pressures cost a lot to produce and maintain. That means that the running costs of your plant are very high. 200 atmospheres is a compromise pressure chosen on economic grounds. The cost of generating the pressure exceeds the price you can get for the extra ammonia produced, if the pressured used is too high.
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