The most obvious answer is the meta one of Oda not planning to stretch the manga as long as he has and running into scaling problems of a similar vein to how Super has constantly retconned Beerus' power. It's most likely he hadn't planned on making the New World prior to Moria's introduction when he'd originally planned for the manga to only go on for 5 years and has made it apparent that a lot of the New World material is being made up as he went along such as how nobody showcased the telltale signs of Busoshoku enhancement prior to the timeskip despite the so-called "War of the best" being the best time to do so. Even Kuma treat Lucci's defeat as something thought to be impossible, despite CP9 Lucci being mediocre by New World standards.
The most likely explanation would indeed be not planning so far ahead, with Oda later introducing CP0 as a means of a soft-retcon so that CP9 were only the greatest assassins by Grand Line standards. That works somewhat since, even without Haki, mastering all the Rokushiki techniques would make up for it when some of them offer traits of Haki use (Tekkai and Kami-e) and even though the Vice Admirals have shown use of Rokushiki, none have shown to have mastered all six techniques or use the Rokuogan.
