LN Spoilers:
Future chapters reveal the situation with the lower nobility was deliberately engineered by the royal family & the court nobles in order to destroy the power of the country nobles in order to create a centralized totalitarian government. As seen by Angelica, a Duke's daughter, not having a harem.
Also, the commoners are deliberately not being repressed, in order to use them as another way to weaken the country nobles, thus further strengthening the capital's powers.
Of course, once power has been centralized, and there are zero checks on royal power...
That's not counting on all the ways the plan ends up backfiring, like how Marie managed to basically fuck over the entire government thanks to "reverse harem" having legal precedent, or all the country nobles that end up defecting during wartime, or how despite everything Leon ends up doing for Mylene'sthe Kingdom's sake, the situation is so volatile the country could end up tearing itself apart at any moment due to all of the resentment amongst the nobility.