Well, Ryu/Ken is mostly a rivalry that came about through out-of-game means. Ryu and Ken are rivals in the SF lore, yeah, but who cares about the SF lore? It was mostly a Player 1 vs. Player 2 thing. If you play SF2 main mode it doesn't end with a climactic fight between rivals; the rivalry is just sort of 'there'. It's still Top 5 rivalry ever, but yeah.
I think Red/Blue's rivalry is going to hold stronger feelings for people. Your first fight is against Blue, right after he counterpicks your starter. He dogs your steps through the entire game, showing up at almost random moments just to make sure you haven't gotten sloppy. He's got the ridiculous catchphrase that everyone loves. And then, after you beat the Elite Four, he's already beaten them and is the real final boss. That blew younger me's mind.
I definitely didn't grow up on SF so I could be underselling it, but in general I think people just love the Red/Blue rivalry more. Ryu/Ken sort of just exists while Red/Blue had an great buildup and conclusion, if that makes sense.
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Black Turtle did a pretty good job.