As the title says, i want to know the cause/origin of the peculiar political/social institutions of the different romano-germanic kingdoms.
In particular:
-Kingship: plural in the merovingian dominions (but aristocrats were excluded), but monistic in Iberia and Italy (where aristocrats could aim to kingship)
-Succession law: royal dynasties in Gaul, election in Iberia/Italy. Is it only a matter of ill lucked kings, or is it connected to the nature of kingship?
-Where do the councils in Visigothic Spain come from?
-Military: why the Franks were so powerful, while the Visigoths weren't?
Also, i know that the Visigoths were rather tolerant with catholic bishops, so i don't consider religion as a weakness factor. Many battles found catholic subjects of arian kings fighting/resisting loyally against the Franks, for example.
(looking at LSCatilina)