The uncomfortable truth with video games is similar to that of any other medium and of life itself, which is that games do come and go. There may be those who enjoy retro games for sheer nostalgia, but people who have a fondness for old games are few and far between. It's not just that the game gets old, but also being "figured out," which is why they're then passed for newer iterations in the same series or for a new title entirely.
But with Diablo 2, it's pretty much the undying action role-playing game. It's not just that it's good, but also the granddaddy of them all. There's the first Diablo as well, but Diablo 2 was so innovative for its time that it still gets played today. It's not even just for nostalgia, but also for sheer enjoyment, even when there are plenty of other action role-playing games out there these days, including Diablo 3. Those newer titles are mostly inspired by Diablo 2 as well, which is why a lot of people still gravitate to the originator.
When a game gets "figured out," it means its gameplay and the optimal strategies and tactics for playing it have been uncovered. A big part of playing a game is discovering how it works and how to optimally play it, so most games are then relegated to the "back shelf" once its system has been figured out and its content has been thoroughly consumed. It's not that it stops being fun anymore after that, but people would want to move on to other games after that.
This is mostly true for competitive games such as the multiplayer-focused first-person shooter Quake Live, wherein their gameplay has been thoroughly figured out. But the thing with action role-playing games is that much of the fun in that genre is the grind—the killing of monsters, the acquisition of experience, and the gathering of Diablo 2 items. Even when a game like Diablo 2 has been figured out, the core gameplay and the content still have their rewards to be had, even after hundreds of hours of play. There's still room for creativity as you can try out various character builds and different styles of play.
With a title like Quake Live, which only has competitive multiplayer, there's no longer that much room for creativity. If you try playing differently from what's deemed "optimal," then you're actually playing not as efficiently as you should, especially at the highest level of play. That's why a game like that would then be left behind in the annals of gaming history, as good as it was. However, role-playing games are like good books or movies—it's always good to play it again after a while.
That's why even up to now, Diablo 2 has a community that has kept it alive even after what's now almost two decades. That's such a long time for a game to be alive, but it's not surprising in this case due to how that Blizzard quality is so evident with Diablo 2.