The recent launch of space x rocket into space has created more awareness about space project, people who did not know about the existence of space stations initially are now aware of the station, but let me give a quick summary of how Wikipedia describes the international space station (ISS).
The international space station (ISS) is a modular space station (habitable artificial satellite) in low Earth orbit. The ISS programme is a multi-national collaborative project between five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA (Japan), ESA (Europe), and CSA(Canada). The ownership and use of the space station is established by intergovernmental treaties and agreements--WIKIPEDIA.
The cost estimation of the international space station is $100 billion built 400m above the earth and every details of the building was carefully selected and fixed because any mistake noticed in the future will further cost more millions of dollars to fix.
The international space station is built to be able to accommodate astronauts who are going on research into space, not just on the basis of mere accommodation but to have the complete material required to make them perform research successfully and give them a comfortable space as well.
Safety was put into consideration about the initial plan to place the international space station 700m above the earth, the level of radiation at that height is not safe for humans, so 400km sounded like a better option and that is where the space station was built.
Placing the international space station below 400km was also not a good idea, because anything placed below the altitude of 300km will be pulled towards the earth, it is a process known as ATMOSPHERIC DRAG, if this happens the international space station will be dragged towards the direction of the earth. Even while the international space station is placed at 400km above the earth, it still drops by 2km every year and a re-boost has to be performed to return it back up and stabilize it, if this is happening at an altitude of 400km what then will happen if it is placed below that.
The international space station was initiated in 1998 and at that point the cost required to launch a rocket into space was extremely high, and if the rocket was to be launched to an extremely high level of 700km above earth, then lots of millions of dollar will have to be sacrificed, so 400km altitude seem to be the perfect placement for the international space station when it comes to economic value, risk of the life of astronauts which is safety. Everything that could be achievable from the placement of the international space station at 700km above the earth level is also achievable at 400km altitude above the earth level.