It's not a bad game, a large part of the problem is just that it's not the game people were judging it as.

The fallout series when it started out was so heavily roleplay based it was actually going to use the GURPS tabletop system. They wound up being unable to do that for a variety of reasons partly to do with the game being too violent. The makers of GURPS feared a PR backlash since at the time tabletop gaming was still under some fairly focused media attack. So they made their own system to stand in as a replacement, the SPECIAL system. If you've played the original game you can clearly see that influence. Random encounters, supplies, a rouge like difficulty, an unleveled world, and tons of different dialog based on your characters history, reputation, and stats leaving the player plenty of room to determine how they'd like to play out any particular encounter. Some paths were clearly easier than others but it was from the beginning about telling the story of your character, the vault dweller who left to explore the wasteland and secure a new source of freshwater.

We've moved more and more away from that heavy roleplay and story based system with a living world to a less story based experience which focuses more on gameplay and giving the player variety. In Fallout 4 you're not playing a character, you were never meant to be. You are the player, and that's clearly reflected in where many development resources were spent. People's lenses of what fallout is didn't move along with it though. Judged as a roleplaying game, something it's no longer trying to be, it fails spectacularly. There's not much room to fill any role, the story moves along the same way regardless, and who your character is, their stats, reputation, and demeanor, almost never come into play. Judged as a sandbox though it does quite well in giving the player a ton of options with an expansive world to explore and a slowly building tech tree along with settlements and various other time sinks along the way to keep things fresh.

TL;DR The fallout series has changed over the years but public perception of what that series is hasn't changed with it.