As many people have remarked, Elite is a mile-wide inch-deep sandbox, and there's no end game, and there's not a lot to do, and blah blah blah. You can argue otherwise to a degree, but let's just generally accept it because there's enough truth to it to go off.
More importantly though, it can take a long time to do a lot of things in the game, if only because of how much time you spend travelling (including looking up routes, plotting, scooping, and dealing with interdictions).
Many players don't have more than a couple of hours a day, or a few hours a week to invest in the game, so they don't usually get anything major achieved in a single session, or in a week's worth of play for them (mining for a couple of hours to buy an Anaconda notwithstanding).
This leaves many players feeling that there's a lot in the game that they haven't experienced or achieved, and so they remain fairly regular players for a length of time inordinately disproportionate to the time it should have taken to make the progress they have, thereby resulting in a good section of the player base remaining active for longer, giving developers time to come up with new content to stretch that out even further.
Well played FDev. Well played...
(Disclaimer: Obviously I'm not seriously accusing FDev of building in long travel times for this reason. That was clearly the work of the lizard people).