It seems to me that the suggestion that it has been "completely forgotten" is a vast overstatement. Others have already pointed out that several other games have sandboxy world design.
Take a look at the White Wolf games for a moment. Nary a railroad module in sight, but huge amounts of setting information detailing groups in the game world - absolutely ripe for sandbox play! Never mind the reputation that "storytelling" is about forcing the players down a railroad - the most freedom of initiative I've had in games, the most sandboxy experience I've ever played through, has been in White Wolf, not D&D.