I voted for boring, glut, and need. Though if Nifft's complicated option had been there, I'd add that to the list, too.
Definitely not on the list, though, are trade and manufacture. Unless you want to make magic items more rare than they've been in any edition of D&D, ever, then PCs are going to acquire things they can't use or otherwise don't want; it's absurd to think this wouldn't happen to NPCs as well. And if there are a non-trivial amount of +1 swords in the world, then it seems like someone should be able to make them; while it's possible to create a universe in which making a +1 sword is something no PC would do, it probably requires a lot of assumptions that don't apply to standard D&D worlds.