Sorry. If i try to find the answers seriously, the possibilities may be:
- lack of resource and manpower since they have more urgent priorities like rebels and Emperors? While Skypiea fighters are not actively attacking the blue seas and its citizen, commit crimes in Blue Sea, or try to topple Marine Justice?
- Not many know about the Poneglyph and the history? Even if the Marines know, they cant decipher its purpose other than being an ancient relics from the past?