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the reason is that one part of nintendo console owners just wants to play nintendo games and that has nothing to do with quality, they just like nintendo games (as much as other people like blizzard games) and the other part of nintendo console owners are kids or families who want to play together some mario party or so. with nintendo games i mean games which have the "nintendo style", few other games of other companies are similar and also successful on nintendo consoles.
on xbox or playstation, first party games and third party games are much more similar to each other, they can be as good or even better as nintendo games but they are different. someone who likes those games has such a console and someone who likes nintendo games has to own a nintendo console because other consoles only get very few games similar to nintendo games.
there are obviously also people who own a nintendo and another console but most people own only one console outside of gaming forums.
