The United States Constitution limits states from keeping a standing army because the framers wanted all military personnel to be under the control of the US federal government.
Before the US Constitution was implemented, the Articles of Confederation was the law of the land. At this time, states had significant power and the US federal government struggled to have any states follow the orders of the federal government. This was especially true for developing a military, as it was optional to listen to the laws of the federal government.
When developing the US Constitution, the framers knew it was an element that needed to be changed. Now, all military would be under the control of the federal government. This would ensure that the US is ready for war and would have a cohesive unit ready for action.
