I am was a big fan of Sakura since the first episode. Never liked Hinata, though. Even though I was a kid, I still thought Naruto and Sakura should end up together. Sasuke left the village. - I think he missed his chance.
Naruto treated her better in any way. He saved her countless times. I especially remember the scene in which he threw himself in front of her to shield her from an attack. The series was still young, so I saw this as clear evidence that their relationship changed. I didn't know that Naruto did that with every of his friend so this was nothing special
Hinata is that shy and spineless character every anime has. I always thought that works better in Japan, since they appreciate calm and "shy" personalities more than loud and extroverted. But obviously, Hinata's fanbase is not from Japan.
I don't understand why people like her and hate Sakura.
The only explanation I come up with is that they identify with Hinata's personality and see Sakura as a threat. - she is more outgoing, has more friends and is stronger.
Or probably they interpret something into Hinata's character - because clearly, she is a "loser", - and want to see her becoming more useful and winning Naruto over as a sign that everyone can achieve his dreams.
Sakura was just the girl that had everything - the attention of the boys (Naruto), good chakra control and the praise of the teachers. Probably it would be unfair if she got the hero, too. Just assuming what might be the reason...
And that is not my opinion, even though towards the end of the series and with the beginning of the new series, I grew fond of Hinata more. You cannot change her too much, but she matured. Sakura on the other hand...
To be honest, in Boruto, I prefer Hinata. Sakura is so stupidly okay with everything Sasuke does (Ino asks if Sasuke contacted her, Sakura: No, but he is traveling and I'm excited and do my best blabla) that she kind of annoys me. I mean, if that is what makes you happy - good luck. But how about thinking about your only daughter that grew up without a father figure?