https://wccftech.com/the-last-of-us-part-ii-something-important/

The Last of Us felt like a complete experience, despite its ambiguous ending, and it wouldn’t have been surprising to see that the original game would be the only entry in the series. The Last of Us Part II, however, is real, and it’s being developed only because the team has something important to tell.

Speaking with Game Informer, Naughty Dog Creative Director Neil Druckmann stated that The Last of Us Part II wouldn’t have been developed if the team didn’t have anything important to tell. Even when developing the Left Behind DLC for the original game, the team feared that they were ruining the original game. On the contrary, it gave them the chance to flesh out Ellie more, something that will continue in the second entry in the series.