Seems like OP isn't asking about developers who turned to shit, but rather made a game that missed what made the original so great.
Ignoring the obviously bad titles, Fallout 2. Basically every Fallout title after the original sort of forgot what the setting was supposed to be, which was not the '50s, but scifi, but rather a '50s style take on the future, then the apocalypse. The humor and then modern pop culture references I can take or leave, but without that original take, feels considerably shallower.
Fallout 2 was not made by the same people as Fallout 1.
The people who made Fallout 1 went off to found Troika and made Arcanum, which improves upon many of Fallout 1's aspects and has a similar overall tone.
They also made Temple of Elemental Evil and Vampire the Masquerade: Bloodlines, which were different games but also good in their own right.
Fallout 2 is a case of new people making a sequel to a game made by other people, and not grasping the essence of that game.