One reason was the vast population of Free men to choose from.
And the fact that the South was losing and slowly bled white of healthy young men.
Desperation makes “impossibly” very close relations to “most likely”.
Or is that being bled of healty white young men? Lee himself stated his opposition to using black soldiers and his preference for white ones in a letter dated January 1865. But he also saw the need for fresh fodder and reluctantly supported the drafting of slaves and free blacks.