Part 1. Why was it such a big deal?

Well, put it like this. Up until the 4th century Christians were pretty consistent about worshipping a Trinity, but less consistent about explaining how it worked.

Around 318 a problem arose in Egypt, Alexandria specifically - a major Christian centre, between Arius, a presbyter, and Alexander the bishop. Arius' position was basically that Jesus was a created being. Still 'god' but not God. Alexander, in Arius' perspective, was guilty of teaching the equivalent of two ultimate principles, which has its obvious problems for Christian theology.

The dust-up between these two dragged in other major Christian leaders and sees, and coincided with the rise of Constantine the first. Hence, the decision to resolve this at the council of Nicaea.

Nicaea, in 325, did indeed solve the problem of Arius. But it also, in its aftermath, revealed that different parts of the church thought about these underlying issues and how the Trinity worked very differently.

Part 2. Why Nicaea didn't solve the rift.

Essentially Nicaea made a much larger rift visible, and engulfed the Greek speaking church (mainly, though the Latin speakers were not uninvolved) into heated debates for the next 55 years.

That rift put the Trinity debates at the centre of ecclesial affairs throughout thr 4th century. That's why they kept debating it.

The resolution came as a combination of two factors. Firstly, one side eventually made more convincing theological arguments. Secondly, the rise of Theodosius I provided a political settlement that gave imperial support to the pro-Nicene position.

That's my short, typed on mobile version. Feel free to ask follow up questions, this is my main research field.

/u/talondearg hits #1 well, but #2 really deserves a longer term answer as well.

The Church started out heavily decentralized, especially during period where there were empire-wide and/or local persecution of Christians. By the time Constantine protected Christianity and it later became the state religion, the reality was that the bishops and archbishops had centuries-long tradition of autonomy, exacerbated by the linguistic split between Latin in the West, and Greek in the East.

So it may be that the reason the Council failed to heal the split was that it failed to resolve the primary issue the church was heavily decentralized and only started councils when it benefited the Emperor and/or when things had already spun out of control. By the time an ecumenical council was called, met, and resolved, grievances often spun out of control (as later councils often resulted in schisms).