Because fighting a super with "weak" guns while your abilities have longer cool-downs is not fun. Simple as that. Getting only 10% for heavies, 15% for specials and 20% for primaries as damage bonus in pvp against supers sucks. Vorpal should just outright ignore all player super armor so it is actually the perk slot worth, considering it is ca. 80% the time in PvP not useful on your gun to have. In PvE the perk is fine, since the damage numbers are bigger there than in PvP and Gambit. Vorpal is only good on primaries sadly. Look at it like Anti-Titan-Weapons in Titanfall. Playing against a Titan is not often fun, so the Pilot players have Anti-Titan-Weapons that deal ok damage against Pilots and very good damage against the Titans. Vorpal should be the equivalent of that Counterplay Mechanic in Destiny 2.