I notice that, even though there are still fanatics who still play Pokemon Go (including myself), many people lost interest. For the simple reason, this game is incomplete. See why they lost interest:
(In alphabetical order)
1) Battles are not turn-based
I believe it would be better if they followed the traditional turn-based battle system, they used in the Pokemon games. But in order to do this, they should include more moves per Pokemon as I say in #7.
2) No Pokemon Centers
Like you did in the old games. This would be helpful to cure your damaged/fainted Pokemon, if they were a few Pokemon Centers scattered around your area, instead of trying to collect the Potions and Revives you might get from a Pokestop, or having to pay in order to buy some extra.
3) No trading feature
This would make things easier to complete your Pokedex. Trading the Pokemon you don’t want with a friend of yours (or a stranger you meet on the street) would be a really good feature, unlike giving them to the Professor for just one Candy per Pokemon.
4) Pokemon don’t gain CP or Stardust after a battle in the gym
You only gain some small amount of EXP and nothing more. Normally, since you train those Pokemon at the gym (whether you fight against other team or your team), the Pokemon which participated in the battle, should gain some CP and you should also be given some Stardust too, since it’s a material very hard to get.
5) Raising your level becomes difficult after Level 20
To get at Level 21 you need 50000 EXP, then to get at Level 22 you need 75000, to get at Level 23 you need 100000 and so on. This obviously made people feeling disappointed, and losing interest.
6) You cannot fight against other players
Well, you can do this already, but only at a gym. Still the other player might not be present. I mean meeting against another player, and fight against him/her via Bluetooth (or something like that).
7) You cannot learn new moves for your Pokemon
Each Pokemon you catch, have their moves set. It’s a big flow that they cannot learn new moves, depending on their CP.
In conclusion: Pokemon GO unfortunately didn’t meet the standards of the traditional games. So many people felt disappointed and quit.