Because the FBI made a stupid decision in a total catch 22 situation. They took his gun, if they went to court to make it permanent, then he could win and they’d have to return his guns. By not taking the next legal steps, they could keep him from getting his guns. So by not taking the next legal steps, no red flag was actually raised, aka no block for purchasing more guns.
For years people including those in law enforcement has been raising the issue of lack of mental health resources and sensible approaches to gun ownership. The NRA has blocked all of them. They want nothing preventing people from buying more guns. Hell, if the police take your gun, and you have to buy a new gun, it’s a win for the NRA.
To go back to the original post, the rest of the world is very relieved that things aren’t worse in the US at the moment. You do have an absolute mess on your hands, but the mess could have been world-endingly bad if Trump had won a second term. The sheer bravery of so many Americans in fighting for democracy and justice against so many corrupt systems is something I envy as a British person. That bravery is powered by an optimism we don’t have much of. It hurts when your optimism and hope is trampled on, but you are slowly winning in many areas.