>flags, above all, must be simple>But why?
to quote Chairman Mao<Where do correct ideas come from? Do they drop from the skies? No. Are they innate in the mind? No. They come from social practice, and from it alone; they come from three kinds of social practice, the struggle for production, the class struggle and scientific experiment.https://www.marxists.org/reference/archive/mao/selected-works/volume-9/mswv9_01.htm
This Idea also didn't drop from the skies but derives from the practical purpose of flags i.e. warfare. The main practical purposes of flags and standards in warfare are
1. Identification - who is friend? who is foe?
2. Orientation - from a soldiers perspective: where is my commander? In which direction am I supposed to go? from the perspective of a commander: where are my troops? where are there going?
3. Communication - lowering the flag indicates giving up a position, raising the flag taking one.
There is a reason why for example in the American civil war the flag bearer was always the one leading a charge, it was cause this is a simple and effective way to let the soldiers exactly know in which direction to charge.
Getting your flag knocked down is not just a disgrace to the honor of the particular unit and the troop as whole but also a serious military disadvantage leaving your soldiers disoriented and confused leaving them to be an easy target for the enemy.
If you keep these things in mind the reason why flags should be simple and easily distinguishable become obvious.
If flags are to complex it's harder to identify them on the battlefield especially at a distance which could lead to serious mistakes on side of both commanders and soldiers. Be it soldiers which fell behind being unable to find their units during the Battle or the commander or a messenger mistakes unit X for unit Y and give them the orders meant for unit Y which could lead to high causalities and eventually defeat.
The consequences of mistaking the enemy for your own troops or the other way around should be even more obvious.
Just imagine a new American civil war fought with the current US state flags.
Bad flag design kills.
As for communist vexillology re-using the same colors and symbols over and over again:
this serves the first practical purpose of flags and standards in warfare Identification, especially in a civil war scenario not uncommon in revolutionary communist history.
Volunteers of communist militia A will think twice before opening fire on communist militia B if communist militia B waves a red flag and the other way around even if these militias have no ties at all or don't even know of each other.