It's possible that the important players (Fincher, Craig, Mara) lost interest over time. The script for a sequel was rewritten a few times and eventually they just announced a reboot without them involved, which makes me wonder if talks stalled somewhere along the way. My guess is that Fincher wasn't interested in a drawn-out franchise building process, leading to Craig and Mara losing interest too, but that's just me guessing.

When they finally got around to a follow-up (The Girl in the Spider's Web), it was watered down and not particularly impressive. That movie came out two years ago and I don't know if anyone remembers that it even happened.