SC including guest characters is what gathered that "huge" SC2 fanbase in the first place. That, and making the game in all three consoles of that gen.
Nintendo especially, because whether people hate on the Nintendo consoles or not, Nintendo fans are pretty loyal to whatever game is released on there, and there are plenty of Nintendo fans, especially in Japan. Include a guest character, and you'll get bank on a game.
But that's based all on my assumption. It might not be true, but what is true is that a majority of those who LOVED SC2 more than any other game had the name "Link" in that same paragraph.