>>17861311 (OP)One primary reasons and some secondary, but I will not get into all secondary reasons because this is /his/ and who actually fucking cares.
Caracalla's edict, this really screwed the empire and is what caused the crisis of the 3rd century. It diminished the prestige and exclusivity of Roman citizenship. It diluted Roman identity, this wrecked Roman culture and values. It was the absolute worst thing to happen to Rome. It did not generate the tax revenue that these massive punic retard thought it would. It directly lead to the rise private armies because of what the edict did to the decentralizing of authority due to the aforementioned dilution of Roman identity. Literal barbarian peasants became emperor because of this retard and his edict
Secondary reasons
It stopped expanding. Ancient States needed fresh conquest, loot, slaves, people to exploit. This also meant less land grants to soldiers, less revenue to pay these soldiers and with such a massive border, defensive wars became more common than offensive wars. Lots of expense with no new conquest revenue.
Disease/Plague
Even Marcus Aurelius couldn't plan for something like this. Another huge destabilizing factor for ancient States, some plagues wiped out a sizable population and this would have been concentrated more in economically prosperous urban areas killing many people and decreasing tax revenue. Plagues also fucked the military in the loss of manpower and maneuverability.