Orbiter got a lot of freedom with its sandbox nature.
It's a rather hardcore and realistic space simulation that is then self-sufficient for those wanting to play/fly with this kind of sim and very likely contribute a lot to the appeal of the game.
The complete lack of defined objectives or mission or similar gives you basically a sandbox the size of the solar system (and more with some addons) to do whatever you want and achieve your self defined goals, while that sounds rather blend as space is full of void, the actual realism gives the whole thing its interest :
Just wanting to reach Mars require a lot to do in the use of the various MFD available to you (stock or addons) to get a correct trajectory, to make corrections etc...
Then once Mars in view, trying to build a good enough orbit
And from there, how about trying to land on a Mars base, or trying to catch a satellite or space station for refuel, or simply doing an EVA and trying to move around your own ship ?
It's actually quite interesting how a game that does not provide space lasers to shoot things, does not evil space empires duking it out or do not have aliens motherships tearing planets apart etc.. (though i have not followed the addon releases since a lot of time) can keep the player coming back again and again.
Kerbal is not that different, even if the realism takes a second seat, it has other elements to keep the player interest and focus on it