Mot people are annoyed at Rylance because they probably have no idea who he is if they didn't see BoS. Stupid mentality, but there you go. His win was probably the only surprising one of the night (I don't really count Best Song as a meaningful category, as this year's winner proved).
EDIT: I see a lot of people actually expected the Revenant to win BP. I was definitely not one of those people.