Why was the "war between the states" or the "war of the rebellion" called the first modern war? what were the major differences between the north and the south in terms of demographics, economy, and resources?

Many historians might argue this point   First modern war some consider was the Franco - Prussian war.  

The massive amount of technology used.  
Minnie balls, lever action guns, Gatling guns, trains, telegraph, air balloons.  

North had more people, machines, and factories.  They fought with one arm behind their back.  Yale or Harvard had the row club all four years of the Civil War.  

South had more mountains, but relied on other countries to supply their arms. Relied on cotton.  Thought English would join for that reason.  However, England found other countries to buy from.  

I suggest watching the Ken Burns Civil War videos.  Long but very informative.