Answer:
When they stopped conquering people, they stopped taking slaves. Much of the rural economy was built around slavery and there was a pretty firm bias against allowing slaves to have children- the investment in raising a slave child not to mention the loss of productivity of the mother and the possible death or both mother and child during childbirth led slave owners to prohibit much breeding.
Allowing slaves to have children and form family bonds also made them more likely to revolt- humans often fight harder for the rights of their children for their own.
So, without a renewable source of slaves, it became very difficult to run farms and vineyards. Without profitable and productive agriculture, there were less taxes. With less taxes, there were less funds to do civic improvements like maintaining aqueducts and roads and support standing armies.
Emperors decided to raise taxes on everyone to make up the difference which led to the ruin of many private citizens and the marked increase in wealth of the nobility who were often exempt from many taxes (which they voted for, themselves, as senators) leaving them with the funds to buy up neighboring properties and essentially, their neighbors to work the land. Feudalism grew out of a switch from a civic or community focus to a self or family focus.
The Eastern Roman Empire maintained a strong base of taxation and a standing army for a thousand years after the fall of the West, but their economy wasn't nearly as dependent on slaves for labor. They survived by repeatedly reforming their style of government to suit the current needs without ever losing the power of the central government.
Explanation:
