The simplest answer is that he did not intend to reconquer the entire empire. His wars were fought because the situation made them possible, not because he had a grand plan. In Vandal North Africa for instance, a number of important events happened simultaneously: a pro-Constantinople king was overthrown, there was a revolt in Sardinia that needed the usurper’s attention, as well as unrest in Libya, all of which made it suitable and justifiable for an invasion at this specific point. Even so, the proposed invasion was resisted at court, with even the praetorian prefect arguing against it, so Justinian did not even have the support of one of his closest advisers. It was perhaps wise to be cautious, since the empire had just emerged from a war with Persia and Justinian’s regime had been recently rocked by the Nika Riots in 532, which had a very real chance of toppling the emperor. The quick success of the North Africa invasion no doubt led to more optimism, but as we shall see, conquering the province is a very different matter from ruling it.

For now, war with Ostrogothic Italy loomed. The same contingent factors of the earlier war is evident here as well, since the Amal ruling house of the Ostrogoths was crippled by dynastic strife, specifically between Queen Amalasuntha and her cousin-husband, Theodahad. Justinian and his empress, Theodora, probably played a role in exacerbating this conflict, which eventually led to the queen’s murder. In terms of Roman propaganda, she was then portrayed as a pro-Roman queen and so her death led to the declaration of war. Now, it is possible to suggest that tensions between the empire and the Ostrogoths had been growing for a while, perhaps around the end of Theoderic the Great’s reign and the accession of Justinian’s uncle, Justin I, in 518, but there is no evidence (that I know of) that suggests there was a desire to conquer territories beyond Italy.

In Gaul, the Franks and the Burgundians served as friendly counter-weights to the Ostrogoths in the early sixth century, so they were never only the imperial hit-list. During the Gothic War the situation was murkier, since the Franks helped the Ostrogoths in their war-effort, but there is absolutely no evidence for any Roman attempts to go further west, or even any suggestions that they should. Diplomacy, rather than war, would be how I characterise their relationship. Justinian did invade Spain, but it was done as an afterthought and was a relatively small-scale attempt to intervene when the Visigoths were engulfed in civil war – again, I would describe it as an opportunity Justinian had seized upon, rather than anything approaching a desire to reconquer all of the west. By then (the 550s), the empire was facing grave problems and overly ambitious invasions would only ruin the Romans further; a fact that the able emperor would only know too well.

What were these problems? The 540s began with a surprise Persian attack on the eastern frontier, quickly followed by the devastating Plague of Justinian. Resources naturally then had to be routed east to deal with this. Meanwhile in the west, incompetence had led to mutinies and Berber revolts in the years after the initially very successful conquest of North Africa. In Italy, the quick victories of the 530s had likewise faded away, as the empire now faced the talented King Totila. The 550s brought no succour and it was only with great effort and concentration of resources that the wars were brought to an end. Throughout all of this, Justinian had to deal with a number of plots threatening his power, as well as a religious controversy of his own making, as western churchmen loudly protested his intervention into theology in the events leading up to the Fifth Ecumenical Council in 553. None of this made further wars impossible, but it is I think very understandable why there was no desire for conflicts beyond the territories already conquered.

The reign of Justinian is a complex one and I am sure that I missed out many other factors that I could have discussed, but this is I think a decent-ish summary of why the territories of the western Roman empire were never taken back by the east. For a readable summary of sixth- and seventh-century history as a whole, take a look at P. Sarris, Empires of Faith: The Fall of Rome to the Rise of Islam, 500-700 (2011). On Justinian in particular, the best introduction in English is I think still J. A. S. Evans' The Age of Justinian: the Circumstances of Imperial Power (1996), but if you want a more academic tome, check out The Cambridge Companion to the Age of Justinian (2005). From a more cultural perspective and a side to Justinian’s reign that is sorely under-explored, I recommend D. Potter, Theodora: Actress, Empress, Saint (2016). I hope this has been helpful!