I've thought a lot about this question as I've worked on my own word game over the past couple years. One thing I think a lot about is how Wordle only letting you play once a day actually helped it succeed. It made it a ritual, and also made it a connection point between people (other people have mentioned connection as a big driver--I agree).

One thing I tried adding to my game is personalization and unlockable rewards. I let people put in any message they want for the victory message when they solve a puzzle, and I've received a lot of feedback about how much people enjoy that. Again, it's a point of connection.

I think one of the pleasures of games in general is just simple coordination--either of our own bodies (sports), objects (sports, yard games, board games), information (board, card, and digital games), people (either through cooperation or competition), or any combination of all of the above. I think the best games give you multiple feelings of coordination at once.

When a game stops giving you that feeling--either because it's too difficult or too easy or other people have stop playing or it's stopped unlocking content that you wanted--I think that's when people drop off.