Many players (Trainers) whom I've met and know to have played the game but no longer have said that other responsibilities - Pokémon or otherwise - have gotten in their way of playing the game. And that's understandable, since many of the things in the game do take time and effort to develop and they might not be able to devote that. At the same time, it also seems fair to say that they don't have to devote all of that right away, and they can sneak in time and effort whenever possible, allowing them to still play.
Then, it may be that the people they play with have become "pushy" or toxic in their play behaviors, for which I surmise that a lot of those playing right now share the same sentiment, whether or not they've actually let go of playing the game. The obvious way of mitigating this is to find new people to play with or to play alone sometimes, even if either or both of these may not be viable at times due to the way that the game is dependent on social relations. At least, feelings are allowed to heal within that time.
Yet other players (Trainers) find themselves - or are themselves set - in rural areas with fewer people who may or may not play the game, and traveling to more populated areas may or may not be an option. It may actually be worth it to take one or several days off in this case, as is using Campfire and Wayfarer to try to develop the playability of the area - "build it and they will come", as a certain imaginative work would suggest. If things do develop, it might just be possible to continue play on a more regular basis.
Certainly, there could be many more reasons that players (Trainers) let go of playing Pokémon Go beyond and in addition to the ones I've explained above. The point is that it is apparent that many factors can and do cause them to let go of Pokémon Go, even with all the value that it has for enjoying Pokémon. It's just something that players have to understand for others and themselves, and then to be able to mitigate that with whatever they can muster in order to keep them immersed in the game and not out of it.
One year ago: Limiting Pokémon Merchandise Purchases?
Two years ago: The People I Meet in My Pokémon Dreams
Three years ago: Pokémon Go Community Day Classic, 7/9/2023
Four years ago: On Defining Fans by What They Do
Five years ago: Twilight Wings Addendum
Six years ago: Pokémon Primer?
Seven years ago: Picking Battles
Eight years ago: Cosplay Cards
Nine years ago: Ghosty and Me