Because giving slaves guns is a terrible idea from a slaveowner's perspective.

There were proposals late in the war to emancipate and arm slaves to bolster confederate numbers. These suggestions were met with hostility by the Confederate congress.

"What did we go to war for, if not to protect our property?" - R.M.T. Hunter, President Pro Tem of the Confederate Senate. Howell Cobb likewise argued that "If slaves will make good soldiers, our whole theory of slavery is wrong."

The confederate congress did eventually OK the recruitment of african americans into the confederate army, although without the promise of emancipation. Ask yourself this - if you were a slave and were given arms and training and told to fight to maintain your dehumanizing servitude, what would you do?

The slaveocracy of the South answered the question the exact same way you did, that's why they didn't really recruit blacks until the war was basically lost.

TL;DR: In a war precipitated on maintaining the institution of slavery, arming and training slaves was basically an anathema.