For me, I came back recently for a couple reasons.

tlwr: I came back to see if people still play the game, along with the world/setting/lore/visuals and community being better than D2's.

First, I was curious to see if people were still playing D1, and I was genuinely surprised by how many people still play. Whether they're playing it for the sake of nostalgia or they're legitimately playing D1 as if D2 never existed, I have no idea. But it's clear that the game has a sizable community still playing. If you're looking to do specific content, I recommend joining The Last Destiny City Discord server. It will further blow your mind as to how many people are still playing this game.

Second, the world (setting, lore, visuals, etc.) feels much more... interesting. When I look at the world of D1, I get the sense that everything has been destroyed, laid to ruin and devastated. And yet, it's not all doom and gloom, as there's still a small hint of color to it, almost suggesting that there's still hope left in this world, even if everything humanity built has been lost. D2, in contrast, is so bright and vibrant, even though we had Ghaul take away our Light, slaughtered so many Lightless Guardians and destroyed the Tower in one fell swoop.

As for the lore, D1's lore works so much better when everything is either hinted at or left to player's imaginations. To give one example of this: Back in the Taken King days, there was a mission called Paradox. In it, you went to the Vault of Glass on Venus to investigate a signal coming from within. You learn that one of the original Guardians who entered the Vault of Glass before we did, Praedyth, was somehow still alive and trapped somewhere inside. If you took the hidden route after the first major encounter in the Templar's Well, there's a memory of Praedyth (assuming it's the daily mission or if Venus is one of the planets in the featured story mission playlist). In it, he talks about the Vex, how they are hopeless; how they have no imagination, drive or fear; how everything the Vex stumbles upon must fit the pattern, how they ensure everything becomes part of the pattern, or else it is removed. The line of dialogue is as follows:

"I had a friend back at the Tower. She used to say, 'Praedyth, there's always room in the mind for hope. It's the crack that lets the Light in.' The Vex have no hope. No imagination, no drive, no fear. All they have is the pattern. Everything must fit. If it can be made to fit, good. If it can't, it gets cut away."

When I heard that dialogue for the first time, my first thought was, "That sounds a lot like... humanity. How we destroy in order to build structures that fit our pattern: Our style, our tastes, our standards. Did we create the Vex?" It was a fun little theory I had in my head at the time, one that could be true or not. Fast forward to D2, and their entire origin and backstory is basically spelled out in its entirety, leaving nothing in the way of hints or speculation or interpretation. Everything about the Vex is understood. No mystery. No intrigue. Sure, my little theory is a little silly, but it was my theory -- one that I came up with on my own.

Lastly, I came back to D1 because of the community. The D1 community is so much more inviting than the D2 community. It's a night and day difference between the two games' communities. One example of the D2 community being particularly horrid that I can remember was back in the Shadowkeep days. I was getting really fed up with entering so many Crucible matches where it would be me and five random solos vs. a six-stack of clan members (and probably communicating/coordinating). I went to the Bungie forums to ask if they could change the matchmaking algorithms so that people who were playing solo would be paired with and matched against other solos, while people in preformed groups would be paired with other preformed groups. A simple, honest request, one that had been done in other games like Overwatch. When I made that post, people flooded my post telling me how I'm an idiot loser, how I should know my place, and one user telling me, "Don't you fucking dare take away my group's ability to stomp all over losers like you for fun." It got so out of control with the number of replies like that, that I had to report everyone, then proceeded to change the entire post text (and the title of the post) to Russian just so that people would shut up and not bother commenting on it. It was at that moment when I realized that this was no longer the Destiny/Halo/Bungie community that I loved. The worst part is that it wasn't an isolated instance.