User blog comment:Fintin/Poneglyph of Dressrosa/@comment-1595565-20140508155648/@comment-3425840-20140513123620

Robin is the only person who can read Poneglyphs - yes - but that is now. Less than a quarter century ago there were a bunch of people. If a Poneglyph indeed is there (I am neutral about that theory by the way), it become a dud bullet only very recently. Don't forget, the Void Century was 800 years ago - compared to that, the destruction of Ohara is a rather recent event.

So if they could have been used for blackmailing - the lack people with the ability to read them became major a factor only recently.

In my opinion, what prevented people from reading them is the difficulty of traveling in the One Piece world and the fact that they are well hidden in absurd locations.

Side note: Can Robin teach someone to read Poneglyphs? I know she won't but can she?