Why did a disproportionate number of lower class men serve and die for the confederacy?

Because some upper class men were excused from fighting in the war because of the "law of twenty slaves".

Explanation:

The law of twenty slaves allowed some white men of high economic class to be excused from serving the Confederate army during the secession war. This caused men from the lower classes to serve and die in disproportionate numbers compared to men from the upper classes.

This law exempted from confederate military service any white man who owned 20 or more slaves working on a Confederate plantation, or on two Confederate plantations that were less than 8 kilometers apart.