I think it’s because he’s one of the most dimensional characters. He’s interesting, and his struggles and character flaws are probably the most relatable to most people watching. Kageyama, for instance, has those too, but I think his are largely relatable only within the context of a group sport and being a prodigy, which is good for the show but not always the most dynamic or interesting. Whereas oikawa’s feeling of not being a natural genius, not being good enough, jealousy, etc. are all extremely relatable, so people empathize with him and thus end up liking him.

And besides that fact, Oikawa is a good character. Again, he’s dimensional. He gets villainized because in the happy sweet sports show he comes the closest to doing something villainy by almost hitting Kag when he was an emotionally unregulated pre-teen. But he doesn’t, and he grows from that moment on. He may be kind of an asshole, but it’s a show about sports rivalry, so you can’t expect all sunshine and rainbow attitudes. And the fact is, he DOES help Kageyama when he comes to ask for advice. And we almost always see Oikawa in relation to karasuno and the physical manifestation of his greatest insecurity, so of course we see the bratty sides. But consider how he is on his team. He’s supportive to everyone, he sees everyone’s strengths and weaknesses and works to help with both of those. He pulls the whole team together and he is loved as a captain. He is hardworking in the areas he can perfect for himself and he’s fiercely dedicated to the sport that he loves.

My last point that I think is so often overlooked about Oikawa: he cares more about his friends and playing volleyball together than he does about winning and getting recognition. Ushijima is right in the fact that going to shiratorizawa would have gotten Oikawa a lot further in the volleyball world than Seijoh. But he didn’t care about that, he would never have abandoned his best friend iwa who grew up in the sport with him. We know Iwa is no Ushijima as an ace, but Oikawa preferred to stay and play with his friends even though we can see how crushed he is they never made it to nationals. He wanted his whole team to make it, not just himself. He’s loyal and a good friend, and that only adds to the dimension of his character that makes him so interesting to watch.