I would argue that there are three primary reasons the Nicene/Arianism split in the 4th century became so divisive: (a) deliberate entanglement of other bishops, (b) the depth of the theological issue at stake, (c) imperial involvement in the Constantinian and post-Constantinian church. The disputes about Arius and his theology really flared up as a local issue within the Alexandrian church, when Arius, a presbyter, challenged Alexander the bishop about his theology, specifically his teaching about the Son (that’s Jesus) being God, and what that meant and how that worked. Both Arius and Alexander were in many ways heirs to Origen’s theology, but at this point we could say they were heirs to different parts of that theology. For Arius, the Son was ‘god’, but not in the same sense that the Father was God, and in fact the Son was much more like us. That’s both what made salvation possible (since he was a creature like us), and what preserved the distinction of the Father (that there was only one, sole, uncreated Deity). For Arius, Alexander’s position would end up in Two Gods, and any more than One is polytheism.

For Alexander, Arius position basically meant that the Son wasn’t ‘God’ in any real and meaningful sense, and so couldn’t save us at all, and worshipping him would actually be idolatry.

Alexander had the power in Alexandria and in local councils had Arius deposed and exiled. That’s when issue (a) came into play. Arius certainly wasn’t alone in his thinking, and he went to Caesarea, and basically started a letter-writing campaign to try and get other bishops in other regions on side. Alexander responded in kind, and suddenly the issue began to engulf other parts of the Imperium. Not that this was entirely unprecedented, but when combined with factor (c), it really became a major issue.

Turning to factor (c), Constantine hadn’t been sole ruler that long when he had to deal with this issue in the church. He’d already tried to solve (unsuccessfully) the Donatist issue, and Constantine’s alignment with the church meant that he almost couldn’t not intervene in order to secure unity in one form or another. I’ve answered elsewhere recently about possible precedents for why Constantine went down the path of a council, and in this case Nicaea was preceded by a smaller council at Antioch which to some extent pre-empted the issue. The council at Nicaea did effectively ‘solve’ the problem of Arius, but not without creating some other problems. It was clear to those gathered that Arius’ actual theology was beyond the bounds of collective Orthodoxy, but the settlement that arose left a lot of parties ‘unhappy’. In my view, it revealed that theologically speaking the collective church just hadn’t, and hadn’t needed to, think through some of these trickier issues relating to inner-Trinitarian relations and ontology. So you then get large parts of the church throughout the empire not really thinking that Alexander (and his successor Athanasius) are right or that the Nicene creed is a good formulation, and developing in different trajectories. (Some of this is covered in my related answer here.)

I won’t go down that trial right now, except to return to point (b) these really were matters of heaven and hell, in that they were theological distinctions that dealt with the very nature of the God they worshipped, and how salvation worked. Get this wrong and you were a polytheist by accident or a modalist by mistake.

Of course, once Constantine had set the course for imperial involvement and ecumenical (aka Empire-wide) councils, later emperors were to some extent tied into that and also involved in the on-going disputes for the next 56 years. They couldn’t really (except for Julian), exempt themselves from the playing field of ecclesial politics once they’d put themselves on the board.