>>714611620I don't feel like writing an essay, but the answer is starring you in the face, and you seem to miss it.
>FO3 story is literally just "somehow, enclave returned". NV's story is about geopolitics of this random chunk of wasteland. Yeah, about that. There's nothing in Van Buren about fighting over water resources, but it's central to F3's storyline. They took what made some sense (factions fighting over water), and then made it more grounded. They looked around the West coast for something that could fit - Hoover Dam. Then replaced the good guy faction of the Brotherhood with the NCR. Brotherhood are then relegated to their role similar to FO1.
Then a new bad guy faction is formed, rather than rehashing the Enclave, and this is from Van Buren. The Legion occupies the Enclave role.
Compare who wags their finger at you and gives you a warning if your karma is too low or high in 3, to who comes after you in NV. Instead of something abstract like Karma, it's replaced by faction rep and it's given more grounding.
Rangers exist in the previous Fallout games, but they get a massive expansion in New Vegas because they're also inspired by the Reiley Rangers units from FO3.
Just think of someone playing FO3, going "okay this is retarded. This would make more sense instead." and you get New vegas.