I think the hype was just Sandra Bullock + original idea for a horror flick + on NETFLIX?! In 2018 Netflix was still in the early stages of putting out original movies and hadn't yet set a reputation for its originals being shovelware offerings made just to pad its content library. It released Scorsese's "The Irishman" the following year which also bolstered its reputation for a while.

But yeah I watched Bird Box when it came out and found it terribly lackluster, and I'm admittedly a big Sandra Bullock fan (actor, not person).

I assume the excess hype out of nowhere was due to a particularly successful guerrilla marketing campaign. I recall the first time I experienced one: 98 or 99, I come into work and 5 out of 30 people are talking about Blair Witch Project and how they plan to go see it. They must have hit all the right AOL chat rooms or whatever the hell they did back then.