The first reason that we might think is because of zero gravity in the ISS orbit. This is not true. Gravity still exists at the height from above Earth where ISS is orbiting. However, astronauts float in ISS because of something called free-falling.
Think of it like you are on roller coaster and when you just hit the peak of the path going from downhill to uphill, you will feel weightlessness momentarily. ISS in the orbit is in free-fall state all the time throughout its path. But you might wonder if ISS is free-falling, how come it doesn’t hit earth? That’s a good question. So the ISS travels around the orbit roughly at 5 miles per sec. Because of its acceleration force and Earth’s gravity acting simultaneously on ISS, it will remain in its orbit and thus avoiding collision with the Earth
If you remove Earth’s gravity from this equation, the ISS will come out of its orbit and keep travelling farther away from Earth. Instead, if you remove acceleration force of the ISS from the equation, the ISS will free-fall on the Earth’s surface just like an apple falling from the tree.
References