Agree with Trajen777's answer above. In addition to the obvious point about the plague seeming to have done most of the damage, here are a few additional points.
The first is that Justinian actually didn't commit that many resources to the western campaigns. The wars with Persia in terms of financial and manpower commitments always took priority. The only time a really big army was sent west, the one under Narses that finally defeated the Ostrogoths, was after the war with Persia was settled. In fact the only times any real forces were sent west was during periods of peace with Persia, otherwise the pattern was more withdrawal of forces from the west for the Persian campaigns. And as soon as an area was conquered, tax-gathers showed up to make the conquest pay for itself.
In fact, you often read in the same histories that complain about Justinian over-committing to the western campaigns, complaints that he alienated the inhabitants of the newly conquered provinces with overtaxation and that he nickled and dimed his western commanders.
This is a historical pattern throughout East Roman and Byzantine history. The eastern frontier always took priority over the western and Balkan territories, with Italy and Africa usually third after the east and the Balkans, and you only see forces sent past the Adriatic when things are quiet on the eastern frontier. Justinian was no different in this respect.
Incidentally, the forces that took southern Spain were commanded by an Italian and seem to have been raised in Italy. And even in Justinian's religious policies, whenever there was a choice between throwing a bone to the Monophysites in the eastern provinces or appealing to the western Christians, he always tried to pander to the eastern Christians, even though his own personal beliefs were more aligned with the western Christians.
The second point is that the East Romans held onto Justinian's conquests for quite some time. We tend to view ancient history in a compressed way, and fail to realize a year in the sixth century had the same number of days as a year in the twentieth century. As of 600, which was 35 years after Justinian died, the East Romans still held everything except the Lombard areas in Italy. Since the Lombards were a major component of the force that conquered Italy for the East Romans, and the stories about their arrival in Italy have a fanciful aspect, I strongly suspect that the Lombard areas were lands allocated to the Lombards as payment for conquering Italy. The most strategic parts of Italy for the East Romans were somehow not "conquered".
Excepting the Lombard areas, the first major loss was Spain, about 50 years after Justinian's death, and there was a Byzantine foothold in Italy 400 years after Justinian's death.
The third point is that not taking Italy and North Africa would have come at a strategic cost. We can see this by what happened after the Byzantines lost the remainder of their Italian territories. The kingdom the Normans organized there invaded the Balkans and were a major threat to the rest of the empire, as was the successor south Italian kingdoms. During the Ostrogothic wars, an Ostrogoth fleet raised havoc in the Aegean. Justinian invaded Italy when an anti-Roman faction had taken power (the invasion by the way was legally in accordance with the rights the East Roman Emperor retained in Italy with the initial Ostrogothic settlement). An undisturbed Ostrogothic kingdom in Italy would have been a major threat to the empire. The Vandals had been a thorn in the side of the empire and anyway most historians don't contest that the empire profited economically and strategically from the north african conquest.
The first is that Justinian actually didn't commit that many resources to the western campaigns. The wars with Persia in terms of financial and manpower commitments always took priority. The only time a really big army was sent west, the one under Narses that finally defeated the Ostrogoths, was after the war with Persia was settled. In fact the only times any real forces were sent west was during periods of peace with Persia, otherwise the pattern was more withdrawal of forces from the west for the Persian campaigns. And as soon as an area was conquered, tax-gathers showed up to make the conquest pay for itself.
In fact, you often read in the same histories that complain about Justinian over-committing to the western campaigns, complaints that he alienated the inhabitants of the newly conquered provinces with overtaxation and that he nickled and dimed his western commanders.
This is a historical pattern throughout East Roman and Byzantine history. The eastern frontier always took priority over the western and Balkan territories, with Italy and Africa usually third after the east and the Balkans, and you only see forces sent past the Adriatic when things are quiet on the eastern frontier. Justinian was no different in this respect.
Incidentally, the forces that took southern Spain were commanded by an Italian and seem to have been raised in Italy. And even in Justinian's religious policies, whenever there was a choice between throwing a bone to the Monophysites in the eastern provinces or appealing to the western Christians, he always tried to pander to the eastern Christians, even though his own personal beliefs were more aligned with the western Christians.
The second point is that the East Romans held onto Justinian's conquests for quite some time. We tend to view ancient history in a compressed way, and fail to realize a year in the sixth century had the same number of days as a year in the twentieth century. As of 600, which was 35 years after Justinian died, the East Romans still held everything except the Lombard areas in Italy. Since the Lombards were a major component of the force that conquered Italy for the East Romans, and the stories about their arrival in Italy have a fanciful aspect, I strongly suspect that the Lombard areas were lands allocated to the Lombards as payment for conquering Italy. The most strategic parts of Italy for the East Romans were somehow not "conquered".
Excepting the Lombard areas, the first major loss was Spain, about 50 years after Justinian's death, and there was a Byzantine foothold in Italy 400 years after Justinian's death.
The third point is that not taking Italy and North Africa would have come at a strategic cost. We can see this by what happened after the Byzantines lost the remainder of their Italian territories. The kingdom the Normans organized there invaded the Balkans and were a major threat to the rest of the empire, as was the successor south Italian kingdoms. During the Ostrogothic wars, an Ostrogoth fleet raised havoc in the Aegean. Justinian invaded Italy when an anti-Roman faction had taken power (the invasion by the way was legally in accordance with the rights the East Roman Emperor retained in Italy with the initial Ostrogothic settlement). An undisturbed Ostrogothic kingdom in Italy would have been a major threat to the empire. The Vandals had been a thorn in the side of the empire and anyway most historians don't contest that the empire profited economically and strategically from the north african conquest.