Question about Roman subjugation area

[Kongeslask]

I have noticed that the SPQR is allowed Type II government in certain areas which they historically never controlled, like Jutland (Gawjam-Kimbroz) and latter-day Prussia. They are also allowed to "subjugate" Mesopotamia, which historically they controlled only briefly, in the time of Trajan if I remember correctly. Yet in Persia proper, there are only "alliances available". Given that the seleukids have "subjugation available" in most of Persia (and even a homeland province in Persepolis), and that historically the romans were the ones that were most skilled at exporting their form of government to conquered areas, it does seem at least plausible that they would be able to romanise the desirable areas of Persia to a similar degree as Greece, Egypt and the Levant if they had managed to properly secure the area. It actually seems more likely that they would attempt that with Persia than with some undesirable (to the Romans at least) baltic province. What was the rationale behind giving the romans the option of Type II government in some areas where it is unhistorical (and also implausible), but not in others?