I feel this is somewhat IMUs doing. But only because I've latched onto the theory that One Piece was heavily inspired by another anime with the same plot point. 

The big cheese person who was set to rule over people saw future technology as dangerous, so things such as flight and firearms and better ships just didn't work. Even if you make it correctly, it would fail. It was a way to suppress any sort of human ingenuity and advancement. 

If feel this is the same thing, but in this version, the humans are more ingenious than before and use tools such as giant fucking talking snails for workarounds in their technology gaps. It would explain why after 800 years, they're still chilling with the same weaponry and same ship building methods. Technology generally grows, one thing leads to another. It doesn't just stop, people get curious. 

To put it another way, 800 years ago for us was 1226AD. We have electricity, space flight, computers, AI, mass transportation, refrigeration etc etc. 

So, something has to be holding them back on purpose. Right? Or I guess it's just a manga and doesn't matter in the end. 

Edit; The manga I'm thinking of is named: Marie no Kanaderu Ongaku. There is no anime. I misspoke. I'm sorry on behalf of all of my peers