Because if you nerf it, there's no real reason to use Thorn over TLW or most any other legendary hand cannon in the game.
Go the other way, make the other weapons more competitive. The auto rifle nerf IMO put more Thorns into the crucible than the powers and capabilities of the weapon did.
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The point is there shouldn't be a reason to use thorn over any other gun. Likewise, there should be a reason to use tlw over thorn what we want is a balance not to nerf it to the ground.