According to the PHB:
When you take the Attack action and attack with a light melee weapon that you’re holding in one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand.
Given that Unarmed Strikes do not have the light property, this implies that you can't use your bonus action to attack with them. Unless you're a monk. However, why is that?
Mechanically speaking, why can you attack twice with two daggers, but you can't attack with a dagger and then punch, or even punch twice? What is the stated reasoning behind this rule? Is it for balance terms, or is it so the Monks have a defining feature other classes don't have? Or is it even historical?