1. People hate Sylvanas because they disagree with the direction of the BfA story (or they are not getting the deeper lore Pyromancer is talking about - Azeroth is a prison for Azeroth, and so on).
2. People hate Sylvanas because she's a woman and a dominant one at that.
3. People love the Lich King because he was once cool when it was Ner'zhul, Arthas and Frostmourne (although Bolvar is no Arthas and doesn't have Frostmourne).
4. People love Bolvar because Blizzard had been building him up throughout Legion and BfA.
5. Frankly speaking, the cinematography is utter garbage. Face it, it's just a fight between two people. And one of the most basic rules for a good story is that a character should suffer for the pleasure he gets - but Sylvanas does not.
So, in a word, people are just fanboying the Lich King for some reason HARD, and fanhating Sylvanas equally HARD, all tied together with honestly an objectively boring cinematic on par with MoP, Legion or BfA. Throw in some glorious anti-feminist incel rage and blind nostalgia for Arthas (although it's a noname Bolvar insetad), and you have a perfect storm or a witch's brew.