The eerie desolate atmosphere (and soundtrack) was replaced by cowboy yabadabadoo shit and explosions
Only the first Fallout felt eerie and desolate. Fallout 2 was bonkers, and FNV captured that spirit well imo
They shouldn't all have the same tonal atmosphere. They're different games in different periods of the Wasteland.
The world of Fallout was a world just fresh out of a nuclear war, just about picking up and going from pure survival to something more. It was on the threshold to start rebuilding civilization, but wans't there yet.
(also the entire region would probably be in a better state if The Master didn't spend the last decades kidnapping dudes en masse to make a giant army of supersoldiers)
The world of Fallout 2 was a world was finally recovering and looking beyond mere matters of survival.
The world of FNV is about two advancing civilizations meeting and clashing in (and over) a lawless No-Man's Land. Civilization is already back on both sides of the Colorado, its about who is going to win.