Would you go to war to protect feudalism? Or to save the Tang dynasty?
Of course, you or I aren't going to go to war to save something that is not a part of our world.
If slavery was a part of our world -- if it was a part of our "way of life" -- we might go to war.
But the decisions about war for this or that reason are made higher up. A 20 year-old wasn't going to consciously decide to go to war for slavery -- though plenty did for the "Southern way of life" -- but might well fight in a war that was caused by conflicts about slavery.
FWIW, Alabama, Mississippi, and Arkansas, like Indiana, Illinois, and Iowa, had been sparsely settled a generation before. It wasn't uncommon for someone to start out with little, acquire land, build a fortune, and become a major slaveowner.
Why do you think the expansion of slavery was such a major issue. One reason was Southern pride. Another was the hope that one could become a big proprietor in the frontier territories. Maybe the hope was misquided, but that's where pride took over.
In particular, the opening of new land was backbreaking labor, but could lead to very large profits in a few years. The south depended on slavery to do that, and slaves were rented for that purpose.
The R.E. Lee family plantation produced slaves to be sold or rented out, and the very large profits from developing new land would be shared with the slave owner either through rental fees or through high prices for prime slaves.
That was not the method used in Illinois, Iowa, Indiana. Lincoln described the method there well. A man would work for others for a year or two, save his money, and would buy some land or a business, then work for himself, and after that would hire others to work for him.
Lincoln was also the first US president to have a patent on an invention, so he was aware of another approach to make money.