The nostalgia factor is what does it for a lot of people. A bunch of the old fans hate Bethesda for how they snatched away the Fallout IP from Interplay, (Which was a doomed company anyway) and they often ignore that Fallout 1 & 2 were buggy as all Hell. Barely functioning in many places. They also rarely mention the last Fallout game made by Interplay, that being Fallout: Brotherhood of Steel. The most dumbed down and straight up awful game in the entire series.Lol what kind of deep morality did Fallout ever have? They had a freaking Karma system since the first game telling you exactly how good/bad you are. Its a joke to say the game has "tough morality" when it gives you goodboy points if you do what it wants to. The choices where just as shallow in the first Fallout games (and New Vegas for that matter). For example that "nuke an innocent town" had a 1-1 equivalent in Fallout 2. There is a city full of ghouls living around a Nuclear Power Plant. A racist but powerful person wants you to blow them all up to get citizenship in the Vault. You can go in and blow them up or fix the plant so it stops leaking radiation. Both make you progress down the story equally. How is that any better than Fallout 3's example?
Fallout 3 saved the Fallout series from disappearing into irrelevance. It isn't a bad game, it's just that people have rose tinted glasses for the old games, Fallout 4 was mediocre enough that people think poorly of all Bethesda style Fallout games, and 76 was such disaster that it solidified that for a lot of people.