This is because of its orbital inclination. This is the "tilt" of its orbit relative to the equator.The orbital inclination of the international space station is about 50°, so it orbits in a plane that is tilted 50° from the equatorial plane of the earth. Therefore, the highest and lowest latitudes it goes directly over are roughly 50°N and 50°S. It is in a high orbital inclination so that it can be reached from Baiknour in Kazakhstan without spending much fuel going from a high orbital inclination to a low one.