I suspect this Nisean Horse has been already beaten to death, but there is something i cannot quite understand concerning the Pahlavan Reformed Governments.

Every single faction that unlocks a new set of government options after reforms (excluding the Lugiones perhaps, with which i haven't played) obtains governments that both offer strict boni to public order and/or better development capabilities. Furthermore, they have a wide area of availability (looking at Urban Land of the Sauromatae and Satrapies for the Saka and Pontos). Sure, the process may be difficult, but it's well worth it in every single case.

However, when time comes and the Pahlavans transition from steppe raiders into empire owners, the best factional goverment available in most cases is... Parthian Border March.
Which offers both a strict public order malus due to happiness (the bonus to law does not offset this, even though technically for the Persian factions law is more worthwhile) AND paltry building capabilities. My question is the reasoning behind this. I understand that not every single faction has to be identical, but the difference in the reform rewards concerning the lesser factional governments is rather... jarring.

This question, by the way, is not extended to recruitment option changes, just the civic side of the Pahlavan Imperial Reform.