And to your point in this post about why, you have to look at the primary purposes behind the notion of citizen militias - "to prevent the establishment of a standing army, the bane of liberty". {Gerry}.
Standing armies, with professional soldiers, were the arm of tyrants, built upon the ruin of the militias. While great portions of the people were the militia.
And now the new constitution was to give the govt substantial power over those bodies (hoping to make them more effective), it was a big cause of distrust.
Yet, the entire constitution, starting with the preamble, ending with its guarantees, its very existence - is predicated on the role of the people - serving as the state militias - defending and securing their rights, the laws, the states, and the nation.
The militias were EXTREMELY vital, so they had to be effective, and they had to be uniformly & effectively armed (via themselves - so they couldn't be DISarmed by the govt), and organized and trained.
"...in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defense, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity"
"15: To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;"
16: To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;
1: The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States;
"The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence."
"Whenever Governments mean to invade the rights and liberties of the people, they always attempt to destroy the militia, in order to raise an army upon their ruins." {Gerry}