Probably because it's too broad for a tag, even though it relates. "Nintendo" is a company name. It would be like tagging a Halo pic as "Sony" because it's available for the PS3. That's my best guess, anyway.
i.e. it's a tag based on fanboyism. It would be worthless to tag Resident Evil, Grand Theft Auto and Crash Bandicoot as Sony because it's not a group of things that are particularly relevant to each other. But when it comes to Nintendo, the fanboys like one series, they like them all.
Resident Evil, Grand Theft Auto, Crash Bandicoot aren't developped by sony. Same with Halo and Microsoft. These game ARE developed by Nintendo. So, the question is, would it be wrong to tag Resident Evil with Capcom, Grand Theft Auto with Rockstar, Crash Bandicoot with whatever company made it, and Halo with Bungie? I don't really see a problem with it...
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