There was no Edict of Milan.

After Diocletian’s retirement, the Roman tetrarchy that he created quickly began to fall apart. In particular, there was conflict over who would be emperor (Augustus) in the west, Constantine or Maxentius. In 311 though, Constantine and Maxentius both (though not jointly so far as I know), ended the Diocletian persecution of Christians in their respective territories. They were particularly concerned with the return of confiscated property. This is noteworthy because it means that Christians and their churches were legally owning property in the empire before 311. The reason why this was possible is that Christianity had first been recognized as a legal religion that could own property by the emperor Gallienus in 260!

So in 311, all western Christians already enjoyed legal status in the Roman empire. So what happened in 313? With his victory over Maxentius at Milvian Bridge in 312, Constantine became the sole emperor in the western half of the Roman empire. But Licinius was still in control of the eastern half of the Roman empire. Constantine could not issue a formal edict over territory that he was not in control of. Hence, it is erroneous to say that Constantine issued any kind of decree about the legal status of Christians because it was unnecessary for him to do so in his own territory and he could not do so for land that he did not control. In 313, Constantine and Licinius did meet in Milan. They discussed the status of Christians and Licinius agreed to end the persecution of Christians in his territory and restore confiscated property. The documents that we today refer to the “Edict of Milan” in the works of Eusebius of Caesarea and Lactantius are really different versions of a letter sent out by Licinius reporting on what he and Constantine agreed to, but it was hardly a decree in the sense that it is usually thought of. The key here is to see that 313 is not quite the watershed moment in the history of Christianity that it is often reported to be. The years 260 and 311 are just as, if not more important. All of this is well covered by T.D. Barnes in Constantine: Dynasty, Religion, and Power (2013): 93-97.

Now to try to get to your question. Since there was no “Edict” as has been traditionally understood, it’s a little trickier to address it directly.

The Roman empire was famously tolerant of the practice of other religions. Even Christianity did not always face persecution throughout its history before its legalization in 260. The persecutions were sporadic and regional in nature.

It has often been speculated that Constantine saw Christianity as a new way of uniting the Roman empire. This has led to speculation that his conversion was less than genuine, although that is based more on a misunderstanding of him waiting until the end of his life to receive baptism. (He did this because baptism could forgive all past sins, but not future ones. Delayed baptisms were not unusual in the fourth century.) Constantine’s conversion seems to have been genuine as he did take a particular interest in the religion (attending the councils of Arles in 314 and Nicaea in 325). He also became a patron of Christianity, but his gifts came from his private funds, not from the imperial treasury. Christianity did not enjoy any special protections or favors over other religions in the time of Constantine. It should also be noted that Constantine never surrendered his title of “Pontifex Maximus,” which made him the leader of the entire Roman religious system. All of this suggests that Constantine, although genuinely Christian, was not interested in completely upending the Roman social order. Constantine’s first goal throughout his empire seems to have been peace and order.

A good example of this comes from his involvement in at the Council of Nicaea. There, Constantine sat to the side on a stool, as Eusebius reports. This means that he was not presiding over the council. He functioned as an interested layman. In the theological dispute between Arius and Alexander of Alexandria, Constantine thought the differences between the two parties were minute. Constantine’s concern was not theological precision, but that as many bishops at the council as possible could buy into a particular creed, thus creating peace and unity in the religion. Indeed, Constantine called the council more to resolve a dispute over when to celebrate Easter than to resolve theological controversy. This shows that Constantine was interested in garnering peace and order, not in dictating outcomes.

So, to answer the question, all of this suggests that Constantine’s primary concern was peace, unity, and stability. Since legalization of Christianity was not an idea that was unique to Constantine we should consider the pragmatism of Constantine as a key motivation more than his idealism. This would lend itself to the idea that Constantine was interested in tolerance, not a Christian empire. But, from the moment he, as a Christian, became sole emperor of Rome, the clock was ticking on the survival of old Roman religion.

See: Eric Wickman, “Shaping Church-State Relations after Constantine: the Political Theology of Hilary of Poitiers,” Church History (June 2017): 288-293.