The country was set up so the only direct involvement with the People by the Federal government is through the States. The authors understood human propensity to gravitate towards an over bearing central government, thus making all interface with the People through the States. The Constitution is clear that any direct taxation upon the People by the Feds is to apportion taxes among the states via the same formula as for the number of Representatives.
To advocate a flat tax is to overlook this vital precept of liberty.
Undo what Wilson did