Because enough people have high ilvl in pug finder no one has to lower the requirement if they can fill the group easy & fast with high ilvl people.
Example: I made a group for court of stars on my 830-something fresh alt, put a requirement 830+ and took literally the first people who listed and fit the required roles. Didn't see a single low ilvl, lowest after me was an 857 tank, everyone else was 870+ and that was within 5 mins of listing the group. No one even bothered to check my char and comment on shit ilvl.
And since Nightbane is a timed run you basically have 1 per week chance you don't wanna spoil, no one wants to take risks and waste their time. So it's gonna be zerg requirements.