@Fire Fist:
I already explained this, but…
Let's suppose there's high tide on one part of the world. Say... AGUA LAGUNA. Well, if there's high tide somewhere, there has to be Low Tide somewhere, right? Right.
Now, let's suppose that Raftel is below sea level. So when there's anything else than low tide, it's flooded. And let's suppose that when it's flooded, giant seakings nest over it. WAY TOO GIANT for anyone, even Whitebeard, to fight.
So, let's say that periodically (About every 22 years), the tide hits its lowest. This brings the water level below sea level, and brings Raftel above water. Seakings can't fly, so they can't nest on an island that's above water. So they disperse back into the calm belts.
At that time, the pirates can enter Raftel. No sooner, no earlier.
That's the most plausible theory.
The other theory is that the Log pose takes 800 years to set. Meaning there's only one log pose in existence that points to Raftel, and no one has found it yet.
The marines' ships have seastone on the bottom to make it camoflauged to Sea Kings. Perhaps this technology is limited to the Marines because of Begabank, but even otherwise it would just be too lame if sea kings were all that were stopping them from getting One Piece.
Think about Ace… he's the fire user. What Sea King could ever possibly even come close to killing someone who is made out of 100% pure hellfire? No matter how big you are, Ace could let himself get swallowed and burn the damn thing from the inside out, like in the filler part of Arabasta. So no, I don't think Sea Kings is a good reason.
The ponyglyphs seems like the best answer to me. If Robin is the only one who can read them in the whole world, and you need to read them to get to One Piece, then you have a perfectly good reason why nobody got it.
Also, since Gold Roger could write in the Ancient Language, he could've made his own clues and secret texts.