In 1791, why was the Second Amendment thought necessary to help perfect the recently ratified U.S. Constitution? I suggest it was an afterthought to give assurance for ongoing white hegemony.
Many of the Founding Fathers were uncomfortable with a national standing army. They knew that the national government could build up a national army as needed, but they hoped it could be minimized in peacetime. They preferred state-run militias. They knew that volunteers would man the local militias.
The militias could respond to local challenges. There may have been uncertainty that a national force would respond to the ever-present local threat looming in the 1790s.