"We the People of the United States, in Order to form a more perfect Union [...]
do ordain and establish this Constitution [...]"
People are more mobile than they used to be, and we keep most self-governance procedures the same or similar in every state for convenience. We also try to keep them easy for courts to work with. These goals come under "a more perfect Union".
We can require uniformity of the United States district map regarding anything that must be represented
across the country.
This section requires the federal government to address gerrymandering (distortion of districts),
while not losing sight of the differences in economic levels
or state sizes (which led to frozen apportionment),
by ensuring that states' United States district maps represent their proportions of economic levels.
Article IV, Section 1:
"Full Faith and Credit shall be given in each State
to the public Acts, Records, and judicial Proceedings of every other State.
And the Congress may by general Laws prescribe the Manner
in which such Acts, Records and Proceedings shall be proved,
and the Effect thereof."
To be sure that the district map of the United States represents its people's economic levels,
Congress has the power to prescribe the manner of recording districting
(making sure the state U.S. district maps work that way) and its effect (better representation).
Amendment XXVIII (proposed), Section 3:
"Districts will represent economic level and geography,”
This also details the inalienable right to ordain and establish the Constitution to establish justice,
requiring the United States government to correct any distortion of districts
that can unbalance Congress' priorities.
And it details Article IV, Section 2’s Equal Privileges and Immunities clause,
requiring the United States to ensure that the districts in one state reflect its economic balance no worse
than those in other states.
Article IV, Section 2:
"The Citizens of each State shall be entitled to
all Privileges and Immunities of Citizens in the several States."
Amendment XIV, section I:
“[…] No State shall make or enforce any law
which shall abridge the privileges or immunities of citizens of the United States; […]”
Why didn't the Framers include districting in the first place? They were probably concerned that the most powerful families would press for traditional district boundaries, and over time become like feudal lords within those districts. Using economic levels as a factor prevents this because districts won't become static: developments like a new factory affect economic levels, and people are more mobile today.