Granted, eight episodes is only one episode shy of the Season 2 count, and it matches the episode count of Season 1.
Still, the show used that extra episode in Season 2 to expand the world of Stranger Things, give Eleven some character development, and finally find a way to tie her back into the main plot in Hawkins.
Who wouldn’t want an extra hour of Stranger Things action, given how long we wait between seasons?
It stood to reason that Season 3 would get another order for nine episodes.