Because the federal provisions were a product of the suggested amendments submitted by the states. The states all lumped together standing army, military subordination and citizen RKBA into one amendment in their constitutions, it was what they were familiar with.
1776 North Carolina: That the people have a right to bear arms, for the defence of the State; and as standing armies, in time of peace, are dangerous to liberty, they ought not to be kept up; and that the military should be kept under strict subordination to, and governed by the civil power. . . .
1777 Vermont: That the people have a right to bear arms for the defence of themselves and the State—and as standing armies in time of peace are dangerous to liberty, they ought not to be kept up; and that the military should be kept under strict subordination to and governed by the civil power.
1780 Massachusetts: The people have a right to keep and to bear arms for the common defence. And as, in time of peace, armies are dangerous to liberty, they ought not to be maintained without the consent of the legislature; and the military power shall always be held in an exact subordination to the civil authority, and be governed by it.
1790 Pennsylvania: That the people have a right to bear arms for the defence of themselves and the state; and as standing armies in the time of peace are dangerous to liberty, they ought not to be kept up; And that the military should be kept under strict subordination, to, and governed by, the civil power.
The separate prongs of these directives were never considered interdependent (that all must exist for any to be actionable, i.e., that if a standing army does exist, the right to arms of the people doesn't).
Actually, it is clear that the standing army declarative clauses can never actually be brought to fruition. They are merely declarations of inactive principle. It can't possibly refer to state action because the states are forbidden to keep troops by the federal Constitution and it certainly can not be interpreted to prevent the federal government from exercising its supreme Art I, § 8, cl's 11, 12, 13 & 14 powers.
The declaration in the federal 2nd Amendment, "[a] well regulated Militia, being necessary to the security of a free State" is inextricably meshed (philosophically) with, "as standing armies in time of peace are dangerous to liberty, they ought not to be kept up." To the framers each represented the same sentiment . . .
The declaratory clause of the 2nd Amendment only re-affirms what once was a universally understood and accepted maxim; that the armed citizenry dispenses with the need for a standing army (in times of peace) and those armed citizens stand as a barrier to foreign invasion and domestic tyranny (thus ensuring the free state).
So, without a doubt the inactive, dependent declaratory clause can only be said to be a statement of why the AMENDMENT exists and as such it does not create, qualify, condition, modify or constrain the pre-existing right; it only states one reason (the political one) for why the fully retained BY THE PEOPLE right is being forever shielded from government interference.