To ensure the people were armed and could resist the government if it became a tyranny.
To further that goal all abled bodied men were part of the unorganized militia. States, counties, cities, towns and other groups formed organized militia units. And those included, infantry, cavalry and artillery units. Even naval militia along the seacoasts and on the Great Lakes. Each militia designed their own uniforms. Those militias either supplied their own weapons or the local government or some rich militia members bought them. Yes, even the artillery guns were privately owned.

That is in part why the Civil War got off to such a fast start with all the various militias* joining either the Union or Confederate Armies as volunteer troops. The Union army also had units of "Regulars". All those uniforms caused a lot of "friendly fire" incidents in the early battles. Later in the case of the union forces the Regulars wore all dark blue, volunteers wore dark blue over light blue and some special units wore green. Though some units held on to their own uniforms as long as they could.

*Though many of them were little more than social clubs who only practiced marching and drilling. As combat troops, they were mostly a confused, armed mob. Western troops of the Union and many Confederate troops were good individual fighters due to hunting etc. but not much good in coordinated fighting until they learned the hard way. Then add in that the officers only knew the Napolianic and earlier style of formation fighting and mostly kept it up until the end was largely responsible for the awful casualty rates. 2% of the total population of both sides died = 620,000 of just over 31million population on both sides, including slaves.