It doesn't change for a reason. And it has to fit. Outcast armor doesn't fit, Medic armor doesn't fit (it's supposed to be a prototype that never made full production). The NCR is isn't much different for the same reason the BOS armor isn't different, there are only so many pre-war models of the armor. Enclave is an exception in that they had resources, but no one else does. Also, you do realize the NCR armor is salvaged from the war with the Brotherhood?
I agree completely, but obviously the OP isn't big on the lore (like too many people on these forums). For the most part, I think he just wants to look cool.
Aside from that, Bethesda imo didn't buy Fallout because they wanted to, and I quote "keep the spirit of the Fallout universe alive" they want to make money off of it. TES is there creation, so naturally they'd treat it better than Fallout. This is why I'm worried about it going back into Bethesda's hands with FO4.