Fortnite requires LiveGold

First of all I have playstation so that is not always the best option either they have issues which all systems do. Xbox live has always been a pay for service and that will not change anytime soon. 9,99 a month is not that much for having a secure platform.
And if you get gamepass bundled 14.99 is not bad either. Plust you can basically get this free by doing certain rewards or joining groups etc. I am not going to get into the playstation argument because like i said i have both so as I said before playstation
has its issues also .

It is a free game but you need Xbox live to access the online feature of the game

Ultimately I believe it is up to the developer. I also believe that Xbox Live Gold is the best subscription service on the market.

But you need PlayStation Plus to access other basic features like cloud saves which you don't on xbox live,you get access to that for free on xbox.

Know a fair few people who have lost 100 of hours of gameplay because their save became corrupted & they had no back up because they didn't have PS+

A xbox live sub is very cheap considering all the benefits you get & if Microsoft Rewards is available in your area you could get a live sub for free by earning reward points.

Gold is required to play online. Since Fortnite is an online only game, it requires Gold. They are working on changing that for Fortnite and other games, but currently it still requires Gold.