WoW was not as widespread as fortnite is. A lot of spotsman and celebrities are streaming it making it more widespread than other games ever were. Plus the existence of twitch...
[1] https://www.polygon.com/platform/amp/fortnite/2018/9/20/1788...
[2] https://www.statista.com/statistics/276601/number-of-world-o...
You're using the peak from the graph which omits the first 11 years of WoW (graph starts at 2015, WoW launched in 2004), the description to the right of the graph states the peak was 12 million in 2010 (though of course that's still far less than Fortnite's)