Reason No. 1: Former USA
...dipicts the situation within the former territory of the contiguous United States the way it is described in the novel. It does so by combining the State of Washinton, Oregon, California and the western half of Nevada to form the Pacific States of America (1, p. 18); by placing Eastern Nevada (1, p. 42) as well as Nebraska (1, p. 145) within the Rocky Mountain States and by distinguishing on the eastern side between the remaining USA (1, p. 82) and the "South" (1, p. 18).
Being a Nazi-controlled collaboration regime in its own right, the "South" is nothing else but the re-established Confederate States of America from the civil war era. In this map it does include the states of Missouri, Kentucky and West-Virginia, all claimed by the old CSA (2). The Nazis probably would have accepted these claims. It does not include however Texas, Oklahoma, New-Mexico or Arizona.The role of the Rocky Mountain States is that of a buffer state inbetween territories which are controlled by Nazi Germany and Japan. They could not fulfil this role if Germany would be able to send troops through the territory of the allied CSA all the way west to the PSA border. Therefore Texas and the more western states must belong to the RMS to keep both sides apart from each other.
The Japanese are concentrated on the expansion of their empire in South Amercia. They are exerting some kind of influence or control over Brazil, where they are "burning off the jungles" and "erecting eight-floor clay apartment houses for ex-headhunters" (1, p. 20). Nazi Germany is confronting Japan on the continent, making its own conquests and using the U.S. Armed Forces for this (1, p. 46).