The ISS orbit altitude is constantly changing in the range from 340 to 417 km above sea level. It decreases due to the influence of the atmosphere, but astronauts try to maintain the orbit altitude at the level of about 400 km.
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This range of heights is selected for a number of reasons as optimal from the point of view of all the pros and cons. One of the most important criteria for choosing an orbit is the cost of delivering cargo to the ISS and the very possibility of a particular ship flying to a given altitude.
From a financial point of view, the lower the ISS is, the better, since flights to it will be cheaper. Besides, earlier, when astronauts delivered shuttles to the station, its orbit was much lower than about 350 km due to the shuttle with the maximum payload that basically could not rise to a height of 400 km.
On the other hand, the lower the orbit is, the stronger the friction of the ISS on a rarefied atmosphere becomes, the station slows down faster and falls to the Earth, therefore it is more often necessary to correct the orbit, which requires fuel which still needs to be delivered to the orbit. At an altitude of 300 km, the friction against the atmosphere will be too strong, it will take too much fuel to correct the orbit, and in case of engine failure, the station may not have time to repair before it falls to Earth.
Thus, an orbit in the region of 400 km is optimal: there are all conditions for space experiments, cargo delivery is not too expensive, and not many resources are spent on maintaining the orbit. An increase in the orbit altitude of the ISS, say up to 600 km and higher, will not bring any benefit in terms of scientific experiments but will cost much more.
In addition, at an altitude of more than 500 km, the level of radiation begins to skyrocket, which is harmful to astronauts and station equipment. It will be necessary to create additional station protection, which again will require huge financial injections. Therefore, nobody also thinks about raising the orbit of the ISS.