There are many reasons, but one key reason is solar radiation. The current orbit of the ISS means that is mostly protected from harmful radiation by Earth's magnetic field. If you moved it away from Earth, you would not have that protection and the station would become very dangerous.
Another reason is resupply. The station can only run for a few months or a year without resupply, and the further you move it away the more difficult and expensive resupply becomes.
There are a host of other reasons, but they all stem from the fact that the ISS was engineered to always be close to Earth, and the space environment elsewhere is sufficiently different that the ISS basically can't work in a dramatically different orbit.
Thanks - those make sense. I wonder if there is any opportunity to keep it in use or relevant. We spent $140 billion putting it up there, so if its not viable to spend 2% of that per year keeping it going then I understand the need to deorbit.
Yeah, we can keep B-52s flying for 50 years with insane numbers of launch-land cycles, I'm pretty sure the ISS can fly for a very long time. SpaceX can already resupply it. I don't see why they wouldn't take it over.
B52s can be completely disassembled in a shirt sleeves environment and we've barely explored on orbit construction. The ISS can't be stripped down so far which is actually causing an issue with bacterial growth in places that can be reached on orbit to be cleaned. At a certain point things that are hard/impossible to reach will reach the end of their lifespan and the inhabitants are in too much danger.
How about moving it further up, but making the station completely unmanned, run by robots. They don't even have to be autonomuous, due to the low distance they can be remote-controlled -- fewer resources needed to keep in orbit, fewer resources needed because there aren't any humans. Plus the main inhabitants don't have issues with the vacuum of space, so many experiments could be parked 'outside', maybe on those lattice structures.
Another reason is resupply. The station can only run for a few months or a year without resupply, and the further you move it away the more difficult and expensive resupply becomes.
There are a host of other reasons, but they all stem from the fact that the ISS was engineered to always be close to Earth, and the space environment elsewhere is sufficiently different that the ISS basically can't work in a dramatically different orbit.