It’s not that much about scariness but about the sensible writing and good cast ensemble. The performances are believable and you actually find yourself caring about the characters and their fate.
Unlike most horror movies where every character are singled out and killed until the lead protagonists finds a way out, here the whole family suffers the horror together and fights back. The investigators too become a part of the family.
Another thing that helped was the investigation angle. The whole process gave a much needed realistic feeling absent in most horror flicks. There are other reasons that aren’t coming to mind. Thumbs up