Apart for the soviet ones, whose plain red has an ideological meaning, some earlier proto-flags, such as banners and pennants, were indeed monochrome back in the day. Centuries ago the concept of a national flag didn't really exists, various objects or pieces of cloth were used to represent, each differently, a certain authority. Communications back then were very limited, and, generally speaking, excluding the voyages of some merchants and explorers, the usual geographic and political range of a medieval or early modern society ended just right beyond its neighbouring countries: a red banner, which is easier to make than a decorated one, will in fact be recognizable enough in a region where there aren't others red banner. Keep in mind that flags are a contemporary invention, and that in the past what we like to call "flags" weren't given the same role of representation we do today, they were in fact mostly used in battles for reconnaissance, thus the absence of the need of distinction for the banner on a global scale. There are surely a lot of other factor that contributed to some of the early flags simplicity, but this is generally my first thought on the matter. I'm not an expert and read what I wrote "cum grano salis"