What happened to good mainstream horror movies?

If we want to strictly consider mainstream to be studio pictures then I'd mention The Conjuring 1 & 2, IT 1 & 2, Halloween '18, The Invisible Man. These were all really big hits with audiences and critics and made a lot of money. Honestly though, I think it's pointless limiting mainstream to that criteria. Even though movies like It Follows, Green Room, The Witch, The Babadook were distributed by smaller labels, they still had large theatrical releases and have large fanbases.