Post-apocalyptic, as in post nuclear, genre is typically defined by ruined, half-ruined and abandoned buildings or structures mixed with new, restored, often partially, rougher-looking constructions (perhaps with some exceptions), moody skies, radiation areas and gas masks, among the most common themes.
Go to the images section of a search engine and search for "post-apocalyptic". Notice how even with all the different examples there's a common theme. While at it, search sites like YouTube for "post-apocalyptic music" to listen to a common theme.
Fallout: New Vegas feels mostly like a Western mixed with the 50's mobster theme, where Las Vegas was not even directly hit by nukes—there goes the post nuclear part. The game severely lacks in post-apocalyptic feel. It is even deceptive, considering the context.
Fallout 4 feels largely like colonial America and severely lacks in a post nuclear atmosphere, despite some ruins being present. It feels already too rebuilt.
With Fallout 2, the writers decided to make the Chosen One the grandchild of the Vault Dweller from the original Fallout, thereby necessitating the jump forward in time. However, Fallout: New Vegas and Fallout 4 have no direct connections to either the Vault Dweller or the Chosen One. They both take place in new locations and yet, the writers felt the need to jump far enough forward in time as an excuse for a significant lack of a post nuclear theme.
Fallout 3 had the issue of the setting not matching how many years it's been since the nukes have dropped, among other things.
Fallout 2 has also been described not to feel like the world has been through a nuclear war. It has a similar issue of New Reno, the way it's been written, feeling as out of place as New Vegas, among other things.
Even the original Fallout was far-fetched with factions like Khans. Fallout: New Vegas became even more ridiculous with Caesar's Legion. That's because they lean too much toward a fantasy-like theme.
While it is understandable that people of the wasteland would want to rebuild, if you jump too far forward in time, especially when already changing the location, to where the overall setting looks too rebuilt, then you are no longer quite in the post nuclear genre. The series are called Fallout, after all.
So far, the Fallout franchise has, essentially, driven itself into a dead end from potential post nuclear sequels because it is stuck in a pattern of jumping forward in time while already changing a geographical location.