Let's be honest here, it's more accurate to say they saw the massive popularity of PubG, copied the gameplay mode, and made it free in a transparent attempt to boost a failing game and get in front of the PubG juggernaut. The initial version of Fortnite BR wasn't that great, it was just free.
Where the Fortnite team really did a good job was taking that initial BR version and turning it into something with huge global appeal and continuing fresh content to keep players engaged for multiple years.