https://www.pcgamer.com/fortnite-save-the-world-will-not-go-free-to-play-in-2018/

In April, Luke Winkie wrote about how Fortnite PvE fans feel about Battle Royale taking over the game they love. It's true: Save the World is often overshadowed by its BR counterpart—something which might subside somewhat when the former also goes free-to-play. Epic has announced this however won't happen this year. 

In a blog post, the developer says it's working on a "broad set of features, reworks, and backend system scaling" that it believes are crucial for going free-to-play. At present, these aren't ready, and while Save the World has grown since launch in July 2017, "scaling up for the legions of player heroes who will be joining the fight is key to providing an excellent experience". As such, Epic is in no rush to strike this balance right.