I can say I sort of know why since it's not a secret. I'm guessing they do so because the data they obtain indicates some weapons are simply OP compared to others, or the defenses in place to counter them are insufficient. So you may ask, then why not buff defense? Ah, but that may (would?) buff the targets vs every other weapon as well. I'm not sure they can buff a target against only one weapon archetype such as hand cannons, and not others. Therefore they have to nerf OP weapons. Again, that is just an assumption on my part. Good question though! And to me, a good question is more important than a good answer. However, many people don't ask the right questions. And you can't give the the right answer if you're solving the wrong problem.