This is a little free-form for me, but here goes...
In the game Fate, they mention that PC's have to be pro-active. The implication, as I see it, is that the players have to continually engage the story. That suggests that it could happen beyond a compelling reason. They're curious, heroic, greedy--something that continually makes them snap at the carrot when it is dangled in front of them. It's an overt reference to one of the unwritten social contracts of gaming: go with whatever story the GM has cooked up.
Think about the opening scene in the movie Shoot 'Em Up. Clive Owen's "Mr. Smith" is just sitting at a bus in the opening scene stop when a woman runs by being chased by a hitman. Owen appears to almost sigh slightly, then get up and get involved. You can almost see the reluctance in his mind as he says, "Oh this? This is the story." The movie's a deconstruction of the genre and is sort of hipster-ironic, by the way.
I'm in this mode right now where I'm kind of off "story" RPG campaigns and thinking more about "sandbox" options. There's lots of reasons, but the biggest one is that I'm really aware of how little latitude can sometimes occur in story games. It almost becomes this subtle by-play of power between the GM, who has some idea of where it is supposed to go, and the players who are alternatively exercising their creative muscles and flailing about trying to work out the hidden plot.
But sandboxes imply the need for an economy, a reason for people to head into dangerous locales. The economy could be as simple as money: the PC's want to eat and have a place to live and don't want to spend decades toiling in a medieval or similarly financially disproportionate society in squalor. So, they go do some very dangerous things for a big payout.
Or it could be something more heavy-handed like "exploring this region of space is your job because you're Galactic Surveyors, etc." I don't know. What I've been wondering about is the possibility of creating an economy, or even a purpose that motivates a group to explore a sandbox that is different from simple financial gain but isn't as thin as the assumption that PC's will be proactive.