OK, I'll try.
Since the founding of the nation, the structure of Congress has been driven primarily by accommodations to the wealthy oligarchs of the day.
Originally enslavers, they got more seats in the House. As states were added, they were typically admitted in pairs to allow the oligarchs in the south to maintain their power.
It was never convenient for the large black populace of DC to get even minimal representation in the Senate, so here we are.