Social requirements of theme system

Oct 2011
I had been thinking, why did Roman Empire manage to develop the thematic system of defence, while later (in 15th century) states such as Hungary-Croatia and France had to rely on, at best, small mercenary army reinforcing feudal levies? And why did no Western European country develop it earlier? Thematic system developed by imperial estates being granted to soliders - these estates disappear from the records after 7th century, so whatever the form, they were transitioned to military lands. So any king with sufficiently large estates could have formed such an army.

Few reasons I have thought of:
1) Lack of literate personnel. Byzantine thematic system still required regular updates of muster rolls, and a lot of administration. This required literate administration, and developed bureocratic and educational systems.
2) Inertia and lack of opportunity. Once feudal system had been established, it was hard finding lands which could be granted to form the royal army. French kings, until absolutism, were really kings of Paris. It is unlikely any of Western European kings had sufficient royal estates to form army in such a manner.
3) Politics. Unlike what may be commonly believed, Byzantine Emperor was not an absolute monarch. But by the time Western Europeans stopped being illiterate barbarians, many European monarchs were. So lack of development of such a system might be the same reason as why later Byzantine emperors allowed it to rot: mercenaries may be **** at defending the country, but are very good at defending the ruler. Arming the populace, however, would have really limited the opportunities rulers had to rob them naked.

Any more I have missed?