Why is it that the http://fallout.wikia.com/wiki/T-4x_Power_Armor model is more available in the Capitol Wasteland, and the http://fallout.wikia.com/wiki/T-51b_Power_Armor model is more available in California?

... Well, the T-51b meant to replace the T-4x so the BoS simply used the best technology for the Paladins use, the Mariposa Base Garrison was likely outfitted with T-51b armor to start with and the BoS descends from that Garrison members and their families

The Capitol BoS (and outcasts) use the T-4x models because ... well I guess their T-51b simply became too worn out during all those years and the T-4x seems to been easily found.