WoW has decent pvp that a lot of people watch, many channels are dedicated to wow pvp.
It also depends on who's streaming, when asmon streams FF you going to ff is obviously more popular because it has 50k viewers?
FF is also a LOT more boring to watch, a lot. It's story based, there is very little combat when people are leveling
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Fix the problem, wow did it with legion, but still keeps doing stupid squishes.