I think Frozen 1 being unfinished actually HELPED the fandom grow. You can sense when watching the movie that there's a lot of room for improvement in multiple respects: in the characters, in the setting, in the lore. Well, what does 4chan love to do, but take unfinished settings and fill them in? It's one of this website's favorite pastimes. I think that's a big part of why /frz/ was such a behemoth when the movie came out.

It's just like how the Horse Show generated so much content, especially at the beginning. For the exact same reason: it was an intriguing setting with good characters but there was much that seemed ripe for exploration. You'll note that as the Horse Show went on, and more of its blank pages were filled in, the fan content diminished in volume, or more of it became explicitly AU.