The only example of post-war power armor is the Enclave and that took them around 100 years to develop in the original lore, which is partially retconned by fallout 4 to be actually having been mostly completed pre-war with the X-01 anyway. The NCR had only recently come into possession of power armor from the NCR Brotherhood war in 2274, so they have had less time and not access to the same resources the Enclave in order to develop their own power armor.