No it wasn't. Did you not read the letter Lincoln wrote? He specifically says the war is not about slavery, it is about "Preserving the Union."
You can't have a War about Slavery, when the disposition of Slavery is optional. The war was about Independence, because that was the one thing which was NOT OPTIONAL.
Look, I get it why you want the war to be about Slavery, because supporting a war to crush other people's independence does not sound so very nice in your own ears, but the fact remains that Lincoln was going to continue slavery in the South, but he was going to stop their Independence.
The War was about Independence. Yes, their reasons for seeking independence was to protect their economic interests in slavery, but the reason the North fought them was not to end slavery, but to end their Independence.
Their independence was costing the Union a fortune. The Union was fine with slavery so long as they were reaping some significant profits from it.
Not so. The 1800s was a seemingly never ending series of political pretzel logic compromises between the abolitionist North and the slaveholding south. This state goes in slave, this one free, this other one can vote on it. Slaves who escape to the north are free, no they aren't. Trying to keep the union together while satisfying both sides was leading to things like Bloody Kansas which illustrated the futility of this sort of patchwork compromise. Since you cite Lincoln, I will as well. In 1858, well before he was elected, he said:
"I believe this government cannot endure, permanently half slave and half free. I do not expect the Union to be dissolved; I do not expect the house to fall; but I do expect it will cease to be divided. It will become all one thing, or all the other. Either the opponents of slavery, will arrest the further spread of it, and place it where the public mind shall rest in the belief that it is in course of ultimate extinction; or its advocates will push it forward, till it shall become alike lawful in all the states, old as well as new, North as well as South."
So in 1858 Lincoln said the union would stand but that one side or the other would win out. Of course in light of the provocative nature of the issue and the decades of failed efforts to resolve it politically, this must have seemed pretty unlikely without some decisive event to force the issue to one side or the other. And the war turned out to be that decisive event.
Slavery was what created the wedge that created the war.