It goes deeper than that.

"poor people do not vote but get conscripted to wars" was the primary argument to universal suffrage, to transform republics into democracies and start our bloated leviathan-states.

today, the problem is gone, with voluteer armies, but the "solution" remains.

and limit voting rights is needed to limit state size.

and... on the other hand, being a private is never going to attract many upper class boys. But it may be a stable honorable job with various advancement oportunities for a poor boy. I knew some privates. the most industrious ones finished various courses, or even went to uni, and exit the military as sargeants with diplomas and practical experience. The lazy boys remained privates or corporals doing dumb office work.