Because it keeps falling down to earth due to the (very thin) atmosphere at that altitude slowing it down. To counteract that periodically it's boosted by a vis

Because it keeps falling down to earth due to the (very thin) atmosphere at that altitude slowing it down. To counteract that periodically it's boosted by a visiting Russian spacecraft, however that won't last forever. American spacecraft don't dock in the right location to allow the boost. On top of that, old things require constant and increasing costs in maintenance. At some point it becomes more economical to build a new one than keep an old one running. And in any case the value of keeping it running has to outstrip the cost of keeping it running, with the cost increasing, and the value not (It claims it's international, but China isn't involved, they have their own space station. With blackjack and hookers), and Russia can't afford it. By 2031 SpaceX will hopefully have an orbiting Starship, that craft alone has an internal volume around the same as the entire Space Station's pressurised volume. If Starship is successful, a new station based on launching and docking skills built through the starship program, will be far better value for money.