Middle Ages: What’s so difficult about maintaining a standing army that the Romans could but their successors in the Middle Ages couldn’t?
Standing armies are very, very expensive. You’re taking a group of fit, healthy young men – the people who normally would be the most productive part of your workforce, engaging in agriculture or industry – and getting them to stand around in barracks all day not producing anything. Not only that, you have to pay them for that privilege, and supply them with food and clothing and everything else they might need.
Modern societies can keep a standing army because with mechanisation and industrialisation, we can produce vast surpluses of goods, and thus support a significant number of non-productive people. In the modern USA only 2% of the population is engaged in agriculture, yet they manage to supply food to the other 98% of the people and even produce a large surplus for export. In mediaeval and ancient times, it was more like 90% of the population had to engage in agriculture – and the remaining 10% had to provide all their society’s blacksmiths and carpenters and weavers and boatmen, and merchants and lawyers and priests and nobles, as well as soldiers.