This is what I'm talking about. I would have liked to see evidence of these smaller population centers. In S1, we see barren desert, a few weirdos, a junktown, and nuked Shady Sands. Where are the farms, the outposts, the trading routes, or the tertiary settlements that would have naturally come from proximity to civilization? Only Shady Sands got nuked, where's all the other evidence for civilization? That's what I mean about no evidence of NCR. It's not that I wanted to see the faction I know, I just think there should be more evidence they existed in that region.
Given the entire first season takes place around Los Angeles, probably not there. California supports such a large population in real life thanks to water from the Colorado River, the use of which has turned lush valleys into toxic dustbowls and destroyed rich wetlands. Combined with being badly nuked, the whole region probably wasn't the most habitable thanks to it now having irradiated toxic dust storms.
This is even shown in the show with the Shady Sands residents efficiently tapping into the same water resources as vault 33, the act of which was what inspired Lucy's mom to leave in the first place. The area also wasn't shown in Fallout 2, was overrun with super mutants in 1, and was likely underdeveloped compared the settlements in the San Joaquin Valley. My guess is there is more civilization near San Francisco and Sacramento, as that area is an irl breadbasket with its own independent water supply. This is notable because the showrunners were specifically told to avoid San Francisco by Bethesda. Having Shady Sands near Fresno like in the games would make it harder for them to adhere to that mandate.
As far as them possibly choosing another area as the setting, my guess is they simply wanted to tell the story of the fall of irl America, and therefore the fall of the NCR. Rather than building whole new factions that would always be compared unfavourably to those on the west coast, or be stuck dealing with recent lore on the east coast, why not follow the decline of the NCR shown in New Vegas to its logical conclusion? Like I said before, I think this direction is fitting and that the show would have a harder time if it tried to do something else. It would have a harder time keeping the identity of Fallout without the commentary on America.