I imagine it's because they are unimportant to the plot and because the animators wanted to give a sense of a hive-mind mentality.
The ranks above are individualized as they are powerful and worthy of the viewer's (and Ryukko's) attention.
The zero-star students are individualized as well. Along with this, all the slums are very different looking to each other - In contrast, the houses that the richer student live in are very regular and similar to each other corresponding with their uniform class.
A good example of the higher rank's lack of individualism is
When Mako and Ryukko start the fight club, They start to lose their individuality that they once had in the slums and become arrogant, greedy and unloving towards each other. Once they return to the slums, they regain their own personalities again.