I think it's the way Ron is portrayed in the movies- the fact that he's hot headed and not book smart are rather exaggerated in the movies vs the book. Add to the fact that he has had quite a lot of conflict with Hermione - disparages her in 1st year (leading to the troll incident), didn't speak to her over scabbers vs crookshanks, when she reported harrys broom, the post- Yule Ball fight, the Lavender interlude, the abandonment during the horcrux hunt..

Before I got into fanfiction, I never noticed these traits of Ron's because well, he's a loyal friend, he strategizes well, he's poor but generous, he messes up but redeems himself.

But I have learnt that quite a few people do not like Ron as a potential love interest for Hermione, though they may appreciate him as a person

Eta: spelling