I believe it likely that they simply lacked anywhere to expand to. If you look at the edges of the Empire, they did not have many opportunities. They bordered modern-day Scotland, which had bad terrain that would have made conquering it difficult and made the land undesirable in the first place.
They bordered modern-day Germany, where forests made it all too easy for the fierce local tribes to ambush them (see the Battle of Teutoburg Forest).
They bordered the Middle East, but the climate was harsh (desert) and it was already inhabited by peoples that were empires in their own right (The Parthians and later the Sassanids).
The only other border was along the Saharan Desert and that was clearly not an option.
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When Augustus reorganised the empire after taking control, he decided to stabilise the borders along defensible lines of the Rhine-Danube and the African desert, with the eastern boundary defended by diplomacy. Subsequent emperors did indeed make forays into Germany, Britain, Dacia, Mesopotamia seeking ethnic boundaries, or responding to intrusions, but they progressivley went on the defensive, both to concentrate on internal problems, and through realising the limits of the power of the empire and its ability to govern and defend new territory.
The cause of the Roman empire expansion was successful military campaigns. The effect was that as the Roman empire grew, so did the number of enemies.
Justinian was a Roman emperor and his empire was the Roman empire.Justinian was a Roman emperor and his empire was the Roman empire.Justinian was a Roman emperor and his empire was the Roman empire.Justinian was a Roman emperor and his empire was the Roman empire.Justinian was a Roman emperor and his empire was the Roman empire.Justinian was a Roman emperor and his empire was the Roman empire.Justinian was a Roman emperor and his empire was the Roman empire.Justinian was a Roman emperor and his empire was the Roman empire.Justinian was a Roman emperor and his empire was the Roman empire.
The Church began suffering its own internal problems, and could no longer afford the funds, resources and time to govern the Roman empire besides its own.
The eastern part of the Roman empire was known as the Byzantine empire.The eastern part of the Roman empire was known as the Byzantine empire.The eastern part of the Roman empire was known as the Byzantine empire.The eastern part of the Roman empire was known as the Byzantine empire.The eastern part of the Roman empire was known as the Byzantine empire.The eastern part of the Roman empire was known as the Byzantine empire.The eastern part of the Roman empire was known as the Byzantine empire.The eastern part of the Roman empire was known as the Byzantine empire.The eastern part of the Roman empire was known as the Byzantine empire.
The Byzantine empire was the Roman empire, merely its eastern counterpart.