There is a long story there - after the War Between the States, there wasn't much interest in things military after the bloodbath of that war, and the ex Confederates were not allowed to join a militia unit unless they took the loyalty oath. So, the militia atrophied.
Come the Spanish - American War in 1898 and this lack of militia preparation in particular, and overall military readiness in general became apparent. Most militia units were equipped with Indian Wars single shot Springfields, and blue wool uniforms, while modern armies had transitioned to bolt action rifles.... Additionally, most officers of militia units were very old and not particularly competent. For example "Fighting Joe" Wheeler had the distinction of being a Major General for the Confederacy and later for the United States in the Spanish - American War. The Civil War vets were the only guys who had ever been part of a force sized larger than a regiment and had some idea as to how to organize an army.
This bad experience in mobilizing for modern warfare led to the Dick Act, which established the Army Reserve and the National Guard. The Guard tried to preserve some of the militia aspect by being available for state service, but it was clear that the guard was a federal organization in that it set the standards for officer commissions in place of the states, and in the 1980s, cases went to court about call ups of the guard over the objection of the governor, which made it clear that the National Guard is not the militia of the states, but is separate from a state militia.
Some states have state organizations, but they are haphazard organizations, for example the TX State Guard does not carry arms (with the exception) of one designated "reaction" unit. Thus we are left with the unorganized militia, and any attempt to revitalize it, would require state recognition of a unit, and issuing commissions to officers of the militia.
