What reasons have the designers given for why unarmed strikes aren't 'light melee weapons'?

Question

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?

Answer 1

Mike Mearls answered a tweet related to this question a few years ago in the context of using unarmed strikes for Two Weapon fighting.

there was a reason why we didn't do that, but can't recall. Probably ok if players don't abuse it w/2-handed weapons

Clearly they did have a reason, though it has been lost, at least to Mearls. Based on his concern for abuse, I would surmise that it was a balance choice, but what conditions they were concerned about being exploited are beyond me.

From this however, we can determine that unarmed strikes not synergizing with two weapon fighting is not an oversight; it was a calculated decision by the design team. And while we can assume that balance was the ultimate concern based on Mike's fear of abuse, I don't know of any other statements made on this topic by the design team.