As a kid, I had the original Pokemon Blue for the GBC. Played it, enjoyed it, found it novel, beat it. Went to an event, got an authentic Mew (certificate is still around somewhere). Not long after, I was gifted Pokemon Silver. Played a bit of it. Didn't find it novel anymore. Very rapidly had this feeling of "I see where this is going and I want off this ride". Gave up on Pokemon, and haven't regretted it even sligh…
From what I can tell the staying power seems to be in: 1) New players. More of ‘em born every year! And, 2) Competitive play, which is a huge thing (I hate playing most games with randos online, personally, but lots of people love it). Like with any multiplayer game (call of duty, say) you need the latest entry or you’ll be looking at a ghost town in the multiplayer lobbies. Plus you get to experience the meta evolvi…
Different appreciations for asethetics apply as well: people who get really into the lore, people who really enjoy specific Pokemon (look at Yellow, or the Let's Go games), people who just want "whatever has the best stats".
And each new game tends to take all of those archetypes into account when creating new content.