Mainly because people are stupid. It usually the same kids who can't stand to see Sasuke being placed above Naruto so they become biased against him & constantly deny/counter, or ignore everything positive said about him, especially if it's against Naruto.
Look at this thread as example, "When did Sasuke stop being Naruto's Rival?"
Indra already posted the page of Adult Naruto saying Sasuke is his rival, yet they still commented saying otherwise.
Since they love Naruto so much, you'd think they would accept his own words, but instead they take their love to extreme levels - they love him so much that they'll deny anything supporting Sasuke positively compared to him.
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I like Naruto too, but damn. & these guys are here practically 24/7. I left here about a month & a half ago, & they were all on, i came back today, & they're the first ones i see.
The obsession. Anyway, as you know there's no need to take these types of guys seriously.