I'd say just [insert some story/lore here] and the chocobo population has increased to the point where we can rent them out to adventurers now! Then allow us to rent them at the city and preferably camps as well. Hell they don't even have to make the lore ingame, just post on the lodestone. Their goal is to attract people back, not provide some well thought out story as to why we can now rent chocobos but couldn't 8 months ago(they were sick?)