A few weeks ago, Pokémon GO received an update that brought more harm than good. Different bugs were reported by Trainers, but one of the most annoying had to do with Team GO Rocket: their appearance would completely freeze the game.
To temporary fix this issue, the developers removed these NPCs. While it solved most of the issues, it also brought a new one: players are reporting that they can't level up because they need Team Go Rocket back.
At the time being, there hasn't been any official statement from Niantic.