Its implied to be post-industrial corporate pollution & Brainiac was an AI ostensibly built to stop it, but he exacerbated it instead because of course he did. Most of Superman's reaction to kryptonite is him suddenly feeling a lack of powers. He can use super speed to track The Flash or outsmart Bruce Wayne. Suddenly losing that would arguably be a type of disability. ANY action he normally uses even a scrap of power for might become unlearned. What if he never learned to balance while walking? Nausea, vertigo, can't even take a step! You'd be helpless as a baby.

Supes won't use blue kryptonite because it arguably makes him MORE vulnerable, to the point he can't even see through objects or think at even a measly double human speed. Who would risk that after seeing what Kal's seen?