Why was kryptonite removed from Superman stories in the 1971 overhaul?

In the 1971 overhaul of the Superman comics, kryptonite was eliminated to address the character's declining dramatic intensity.12

By the late 1960s, Superman’s superpowers had intensified to such an extreme level that his enemies could no longer pose a credible threat, causing his stories to become stale and his readership to dwindle. To revitalize the character, DC publisher Carmine Infantino tasked editor Julius Schwartz with a major revision. Beginning with Superman no. 233 (January 1971), the creative team implemented several changes to restore stakes to the narrative:12

• Elimination of kryptonite: This removed a recurring plot device that had become a crutch for storytelling.2
• Weakening of powers: Superman’s abilities were reduced to make him more vulnerable and his conflicts more meaningful.2
• Modernization: Clark Kent was transitioned into a television news reporter, and writers introduced new villains and a greater sense of realism to the series.2