This is according to Naughty Dog’s Neil Druckmann who in an interview with IndieWire, hinted that the studio could find it a bit hard to justify creating a third entry in the series. According to Druckmann, one of the reasons why the first game was so popular was because there were essentially zero expectations from gamers and the developers could do whatever they wanted.
However, since the game’s release and characters in the game and the world have already been established, making Part 2 was already a challenge.
According to Druckmann, “Finding it with the sequel was much harder than it was with the first game, and going forward it would be exponentially harder to justify going back to that world and finding a way to vary things up. There’s already so many things you’ve seen about the backstory, about how the outbreak happens, so we’d really have to figure out how to create a new experience that matches the emotional impact of these stories and I don’t know what that is. Currently.”
This doesn’t mean that Part 3 will not be happening (Druckmann had previously hinted that they could be looking to work on Part 3 soon), but given the challenges he laid out, perhaps it could take a bit longer as they try to figure out how to best approach it.