If I remember right, they were going to give Chekov a Captain's rank in Generations, then thought better of it. Probably figured everyone would ask "then why isn't he the one commanding the Enterprise-B?"
No, that's not quite it.
Koenig's costume had the Commander's insignia, and he convinced the extras who were playing the reporters and shouting questions to call him "Captain Chekov." I've tried listening for "Captain Chekov" in the opening scene over the last (almost) twenty years, and I've never heard it. It was probably looped out in the editing room.