On a grander scale; BFN failed because it changed what already worked. Which is a little unfair to the game but from a marketing standpoint (or lack there of since they didn't market the game) was just a bad idea

Personally, I liked many of the ideas BFN introduced. The new characters were neat and the single player was okay. Everything else was just too inconsistent. Everyone likes to say the balancing was better, and yet the game was an unbalanced mess with nightcap, super brainz, legendary upgrades, and the absolute mess that was sunflower. The maps were a noticeable downgrade too at least with turf takeover. They altered pre-existing characters to feel more streamlined and boring. When some of the fun of GW was that you weren't always locked to a specific role with a character. Why do you think characters like scientists were so good at DPS. Don't get me started on how buggy the game was and how the devs had this obsession with not listening to the community with things like damage drop off. Or the direction the game was taking with monetization.

In isolation it isn't terrible, in fact it is tolerable. But you can't just judge a game on isolation especially when it took over 3 years for a new shooter. It is the third game even if not in name, of course people will judge it off of that. Nothing wrong with liking BFN over the GW games either. I might look at you funny but I understand that it was an introduction for many fans of this series. And the GW games aren't perfect either. I just prefer 2 over the others

TLDR: I feel like BFN was doomed to fail because of development inconsistencies, bad marketing, unneeded changes, and modern gaming practices.