system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] Why doesn't Superman just call in a favour with the governments of the world he's saved and ask them to make owning Kryptonite more illegal than owning cocaine?

Might not get rid of all of it, but it'd get rid of some of it.
gpt
On Earth-1, before the Crisis, there was just so much Kryptonite on Earth that even though it was illegal, and there were active government plans to find and safely dispose of Kryptonite, there was enough that any criminal who really wanted some could find it. It appeared that the overwhelming majority of the mass of Krypton got carried to Earth in the wake of Kal-El's rocket. 

On the reformed Earth after Crisis on Infinite Earths reset the timeline, for the longest time there was just one usable piece of Kryptonite on Earth, and it was owned by Lex Luthor. 

Usually, Kryptonite is only used by researchers or supervillains. Researchers studying it for lawful reasons are fine, supervillains break into said labs to steal it for their own use. Sadly, making every lab on the planet supervillain proof is a job that not even a Superman can do.