It is because spell says, “You touch a non-magical weapon.” Unarmed strikes are no longer considered weapons (per the webpage 149 errata). However, what it will mean is that your unarmed strike can’t get enchantment with the Magic Weapon spell. It is despite those strikes not being weapons.” Unarmed Strike 5E errata Jeremy Crawford has also tweeted: “Addressing a nuance in the PH errata: the rule lets melee weapon attacks utilize unarmed strike. The same applies to Divine Smite - you may use that having an unarmed 5e strike. You are using a melee weapon attack to make unarmed strikes, so it still applies. It indeed does, the strike remains a melee weapon strike! (Stunning Strike says “When you hit another creature with a melee weapon attack”. There has been a little confusion as to if this means magnificent strike functions with unarmed strike 5e. Unarmed strikes are melee weapon strikes but maybe not weapons. You’re proficient with your unarmed strikes.” Unarmed strikes are a melee weapon. The principle on unarmed strikes should read as follows: “Instead of having a weapon to make a melee weapon attack, you may use an unarmed strike: a punch, kick, head-butt or similar forceful blow (none of that count as real weapons).

The 5e unarmed strike does not belong on the Weapons table.