There are two reasons for this:
Thematically: In lore, modrons only communicate with modrons one rank higher or lower than themselves. So this mechanic reinforces that flavor.
Mechanically: It creates a pyramid structure of power, where 3 tiers of modron in a fight can't all just make the one boss a super foe, the lower modrons can buff up the middle ones, and maybe the middle ones decide to fight or buff up the boss. And so it creates tiers of the pyramid for the players to attack, adding interesting nuance to a modron combat.