Because Sack Boy is totally a hard core blood and gore kind of character.
As far as the "criteria" for 3rd parties, Sack Boy is in kind of a weird place. We have:
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The face of stealth games
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A gaming icon
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A gaming icon
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The face of the arcades
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The face of fighting games
Sack Boy doesn't really have a huge legacy like the others, but his gaming genre is very new. I believe he is the face of level creator games, but level creator games don't really have a legacy, so neither does Sack Boy.
Hmmm. What 3rd party characters do you guys think are a gaming Icon? Well...besides obviously what we already have.