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 evolving, so it’s more dynamic than playing on an older one. They’ve got this whole graded ranking and matching system and a bunch of leaderboard stuff going on.

I only know about the latter because I know a guy who usually spends at least a little time way up near the top of the rankings each time a new one of these comes out. Seems like a pretty large scene.

I think it also attracts many different kinds of players (with overlap between them). Some of the same MtG "player personality archetypes" apply to Pokemon: https://mtg.fandom.com/wiki/Player_type : hyper-competitive players, players who like swinging with the biggest baddest coolest Pokemon they can get, and combo players who like figuring out just the right combination of mechanics if you bring in some move from six games ago plus an item from three games ago plus a new Pokemon that just made the combo possible. Also throw in aspects like grinding for shinies or EVs.

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.