Yep. Because they're both cultural flash in the pans that have been generally forgotten by the public except as mindless, one-note fluff. There is/was no tangible desire for either sequel, despite financial returns on both films. Zoolander 2 flopped hard.
Avatar kick-started the prevalence of 3D, and it's one-note was that we hadn't seen anything like it before. But we HAVE now, and there's a new 3D blockbuster coming out every other week. The uniqueness of it has disappeared completely, and the story lacking to a point where there's not even a viable market for extended canon.
Money doesn't equal popularity. That's why it's so common for sequels to flop.
EDIT - Here, this something crazy to think about relevance in the zeitgeist: Fan Fiction. The top 15 movies on Fanfiction.net, by number of stories written in their worlds, includes Pirates of the Caribbean, High School Musical, and Labrynth. Inception has twice as many stories as Avatar. Avatar has fewer stories written about it than Van Hesling, Four Brothers, and Treasure Planet. You want an idea of how much people don't care about Avatar or the canon expanding, there you go.