Which there isn't a problem with that. But where Mario has continued to grow, despite no more growth in viewership than Zelda. Zelda has just declined and declined. Granted, some of the most popular zelda games to speedrun are awful now (Ocarina of Time for example).
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The game isn't fun anymore, and the amount of runners reflect that. It's probably the same for the other games. But people still watch because they know the name.