"In an interview with Masamitsu Hidaka in 2008, it was explained that the GS Ball was meant to contain a Celebi that was to be the star of a large arc of the movie, so the story arc was viewed as redundant and shelved. The ball was left with Kurt with the hopes that viewers would eventually forget about it. This plan, however, failed, as the GS Ball remained one of the most talked-about anime mysteries until the interview."
-From Bulbapedia