Sonic beats Bowser (and Ganon), plus he wouldn't get SFF'd by Link as much. TRE also mentioned how much the site's still stuck in the 90s anyway (our top five games are probably from the 1990s), so it's reasonable to believe the Mario/Sonic-rivalry still resonates with the site.
If that doesn't do it, think about how much people wanted to have Sonic in SSBB; the idea of getting Sonic to fight Mario pushed Sonic from this...
http://www.gamefaqs.com/poll/index.html?poll=1362
...to this...
http://www.gamefaqs.com/poll/index.html?poll=2762
...so the rivalry's worth something. It's just hard to say if it's enough.
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