Because for a sequel to grab the interest of an audience nowadays, you gotta hype that shit up. Cameron knows absolutely nothing about modern-day marketing. He’s stuck in the 90’s. The reason people hype Star Wars sequels is because they’ve constantly stayed in the public eye with tv shows, comic books, novelisations, toys, etc. Cameron could have easily gone this route after releasing the first Avatar, thus cementing it further as a staple in American pop culture, but instead he did nothing and people collectively forgot about it because aside from pretty graphics, the movie itself is creatively bankrupt.